Why is there no archaeological evidence that Christians existed for 200 years after 70 AD?

The lack of evidence of early Christianity has been noted by many scholars.

Edward Gibbon, in The History of the Decline and Fall of the Roman Empire, said :"The scanty and suspicious materials of ecclesiastical history seldom enable us to dispel the dark cloud that hangs over the first age of the church.".

William Fitzgerald, in Lectures on Ecclesiastical History said: "Over this period of transition, which immediately succeeds upon the era properly called apostolic, great obscurity hangs ...".

Samuel G. Green in A Handbook of Church History said: "The thirty years which followed the close of the New Testament Canon and the destruction of Jerusalem are in truth the most obscure in the history of the Church. When we emerge in the second century we are, to a great extent, in a changed world.".

William J. McGlothlin, in The Course of Christian History said: "But Christianity itself had been in [the] process of transformation as it progressed and at the close of the period was in many respects quite different from the apostolic Christianity.".

Jesse Lyman Hurlbut, in _Story of the Christian Church, said:"For fifty years after Paul’s life, a curtain hangs over the Church, through which we vainly strive to look; and when at last it rises, about 129 A.D. with the writings of the earliest Church Fathers, we find a Church in many ways very different from that in the days of Peter and Paul.".

Other historians make similar comments about the lack of historical material from that period, and how, after a few centuries, suddenly Christianity is seen to be flourishing, with no evidence of how it got that way.

Whatever happened, it was not a smooth transition.

It is beyond the AD 70-220 range of this question, but the Council of Nicaea provides a good idea of what the process must have been like.

Describing the period immediately after the Council, historian Will Durant wrote, "Probably more Christians were slaughtered by Christians in these two years (342-3) than by all the persecutions of Christians by pagans in the history of Rome" (The Story of Civilization).

The Holy Roman Empire's version of Christianity was very different from the original Christianity spread by the Apostles. Once "Christianity" became the official religion of Rome, anyone practising anything resembling original Christianity was called a Judaizer and persecuted as a heretic.

It is hardly surprising that under such circumstances, any historical records that contradicted the new official version would have been suppressed and destroyed.

EDIT: As requested, here is some support of the Church's history of persecuting anyone that Judaizes.

From their very beginning, Christians were persecuted by Rome. They were seen as nothing more than a sect of Judaism, and a troublesome one at that.

Norbert Brox, in A Concise History of the Early Church stated:

… the first [Christian] communities were groups that formed within Judaism … Christians believed as before in the God of Israel: their Bible was the Bible of the Jews … They continued to observe (as Jesus did) the Jewish practice of temple worship and law (Acts 2.46; 10.14), and gave outsiders the impression of being a Jewish sect (Acts 24:5; Acts 24:14; Acts 28:22), not a new religion. They themselves probably also simply thought of themselves as Jews

The destruction of the Temple and the extradition of Jews meant that the central body of the Christian Church could no longer exist in Jerusalem. Brox continues:

The Jewish Christians in Palestine had been driven out in the First Jewish War (66-70) but then had returned to Jerusalem; however, after the Bar Kokhba revolt, the Second Jewish War against the Romans (132-135), they had to leave the land because, as Jews, they had been circumcised, and all Jews were now banned on pain of death. So for the moment that meant the end of this [Jerusalem] church.

Christianity spread throughout the Empire, both by deliberate evangelism and by the dispersal of the Jews.

The Church thought of itself as a continuation of Judaism, and gentile converts became spiritual Israelites, accepting the teachings and practices of the Holy Scriptures.

Iranaeus, circa 190, convinced Pope Victor I not to excommunicate Christian communities that celebrated Passover rather than Easter.

Eusebius wrote of Polycrates of Ephesus (AD 130-196):

A question of no small importance arose at that time. For the parishes of all Asia, as from an older tradition, held that the fourteenth day of the moon, on which day the Jews were commanded to sacrifice the lamb, should be observed as the feast of the Saviour's passover … But it was not the custom of the churches in the rest of the world … But the bishops of Asia, led by Polycrates, decided to hold to the old custom handed down to them.

He records Polycrates' letter to the Roman church:

We observe the exact day; neither adding, nor taking away. … [list of saints, including John and John's follower Polycarp] All these observed the fourteenth day of the passover according to the Gospel, deviating in no respect, but following the rule of faith. And I also, Polycrates, the least of you all, do according to the tradition of my relatives, some of whom I have closely followed. For seven of my relatives were bishops; and I am the eighth. And my relatives always observed the day when the people put away the leaven. I, therefore, brethren, who have lived sixty-five years in the Lord, and have met with the brethren throughout the world, and have gone through every Holy Scripture, am not affrighted by terrifying words. For those greater than I have said 'We ought to obey God rather than man' … I could mention the bishops who were present, whom I summoned at your desire; whose names, should I write them, would constitute a great multitude. And they, beholding my littleness, gave their consent to the letter, knowing that I did not bear my gray hairs in vain, but had always governed my life by the Lord Jesus.

The following examples extend well beyond the question's AD 70-220 range, but they provide evidence of the continued persecution of anyone considered Judaizing. It's not difficult to extrapolate this evidence back to a time when the suppression of the practices of the original apostolic church were so severe that they destroyed not only the practice but the evidence of such practice.

When Emperor Constantine decided to incorporate rather than fight the Christian sect, he ran into a problem. Much of Christianity still considered itself to be part of the same religion as the Jews, and so was very resistant to the changes that Constantine wanted to make.

This situation came to a head when it came to replacing the biblical Passover celebration with the Roman Easter. Many leaders of the Christian Church welcomed this change, as it would provide a more familiar event for their members, and would attract more converts. But those leaders that saw themselves, and Christianity, as a more enlightened form of Judaism strongly opposed it.

Constantine used anti-Semitism to silence, and eventually ban, the descenting Christian leaders.

In a letter talking about Jewish Christians, quoted by Eusebius of Caesarea, Constantine wrote:

And truly, in the first place, it seemed to everyone a most unworthy thing that we should follow the custom of the Jews in the celebration of this most holy solemnity, who, polluted wretches!, having stained their hands with a nefarious crime, are justly blinded in their minds.

It is fit, therefore, that, rejecting the practice of this people, we should perpetuate to all future ages the celebration of this rite in a more legitimate order … Let us then have nothing in common with the most hostile rabble of the Jews.

We have received another method from the Savior. A more lawful and proper course is open to our most holy religion … Let us withdraw ourselves, my much honored brethren, from that most odious fellowship

When Constantine convened the Council of Nicaea, he made sure that no Judaizers, members of Christian communities that practised traditional Passover, were invited. But even with a stacked deck, the banning of Passover wasn't unanimous. Constantine easily resolved that issue too. Historian Robin Lane Fox speaks of Constantine in Pagans and Christians:

At Nicaea, the Emperor himself imposed criminal sentences of exile on the bishops who refused to sign. He also investigated other reports of heresy.

This suppression and persecution of Christians that insisted on following the traditions and practices of Jesus and the Apostles continued.

Following the adoption of the Trinity doctrine, for instance, Emperor Theodoseus declared:

We authorize the followers of this law to assume the title of Catholic Christians; but as for the others, since, in our judgement, they are foolish madmen, we decree that they shall be branded with the ignominious name of heretics, and shall not presume to give their conventicles the name of churches.

In 431, Pope Celestine sent Palladius to convert Ireland. There he encountered existing groups that claimed their Christianity was directly descended from the Apostles. The Catholic Encyclopedia records that:

those fierce and cruel men [did not] receive his doctrine readily.

In 595, Pope Gregory I sent Augustine, first Archbishop of Canterbury, to convert Britain. He was successful in the eastern parts, which had a long history with the Roman Empire, but in western Britain he encountered a version of Christianity that had been founded by missionaries from Ireland centuries before. These "heretics" still celebrated Passover.

The Paulician church of Armenia rejected the power of the Church, including mass, communion, and confession. They saw the worship of Mary, the concept of Trinity, Sunday worship, Christmas, and Easter all as paganism, added by Rome. F.C. Coneybeare wrote: "we have before us a form of Church not very remote from the primitive Jewish Christianity of Palestine". In 843, Empress Theodora persecuted the Paulicians, murdering 100,000 and confiscating land and property.

Similarly, groups such as Bogomils, Waldensians, and Cathars, who followed biblical teachings rather than the traditions of Rome were persecuted and slaughtered.

The Roman Church continued its suppression of anything resembling Judaism within Christianity right through the Reformation.

Why is there no archaeological evidence that Christians existed for 200 years after 70 AD?

This is historically not true. To say there is no archaeological evidence that Christians existed for 200 years after the year 70 AD is absolutely baffling to the mind. There may not be a huge amount of archaeological proofs, but they do exist nevertheless.

• The Book of Revelation was written by the Disciple of Jesus, St. John himself and is dated by most scholars to have been composed around 95 AD. The article, "When Was the Book of Revelation Written?" shows clearly that the Apocalypse was in fact written near the end of Domitian’s reign, and that ruler died in A.D. 96.
• "The Last Disciple series is based on an interpretation of Scripture that holds that the entire, not just Revelation, but the entire New Testament was completed prior to the destruction of the Temple in AD 70. In contrast, the Left Behind series is based on the assumption that Revelation was written in AD 95, long after Jerusalem’s destruction. It asserts, in fact, that Revelation describes events that will likely take place in the twenty-first century rather than the first century. This is how Tim LaHaye puts it: “Revelation was written by John in AD 95, which means the book of Revelation describes yet future events of the last days just before Jesus comes back to this earth.” Dr. LaHaye has gone so far as to dismiss the notion that Revelation was written before AD 70 as “historically ridiculous.” A closer look at the evidence, however, reveals not only that such dismissive language is unwarranted but that the late-date position is untenable." - Dating the Book of Revelation
• Many of the Apocrypha (although not canonical writings) were written by Christian during the time frame indicated.
• Epistle of Barnabas is a Greek epistle written between 70–132 AD and is preserved complete in the 4th-century Codex Sinaiticus, where it appears immediately after the New Testament and before the Shepherd of Hermas.
• Acts of Paul is one of the major works and earliest pseudepigraphal series from the New Testament apocrypha also known as Apocryphal Acts. An approximate date given to the Acts of Paul is 160 AD. The Acts were first mentioned by Tertullian (155-240), well within your time frame. Tertullian found it heretical because it encouraged women to preach and baptize. The Acts were considered orthodox by Hippolytus but were eventually regarded as heretical when the Manichaeans started using the texts. The author of the Acts of Paul is unknown and wrote out of respect for Paul in Asia Minor. The author does not show any dependency on the canonical Acts but uses oral traditions of Paul's missionary work.
• Infancy Gospel of James or Protoevangelium of James, is an apocryphal gospel probably written about AD 145, which expands backward in time the infancy stories contained in the Gospels of Matthew and Luke, and presents a narrative concerning the birth and upbringing of Mary herself. It is the oldest source outside the New Testament to assert the virginity of Mary not only prior to, but during (and after) the birth of Jesus. It is from this source that both Catholics and Orthodox get the names of the parents of Mary: St. Joachim and St. Anne.
• Early house churches existed during the times of persecution. There are almost all but non-existent now. The first house church is recorded in Acts 1:13, where the disciples of Jesus met together in the "Upper Room" of a house, traditionally believed to be where the Cenacle is today. For the first three centuries of the church, known as Early Christianity, Christians typically met in homes, if only because intermittent persecution (before the Edict of Milan in 313) did not allow the erection of public church buildings. Clement of Alexandria, an early church father, wrote of worshipping in a house. The Dura-Europos church, a private house in Dura-Europos in Syria, was excavated in the 1930s and was found to be used as a Christian meeting place in AD 232, with one small room serving as a baptistry.
• Dura-Europos church or Dura-Europos house church is the earliest identified Christian house church. It is located in Dura-Europos in Syria. It is one of the earliest known Christian churches, and was apparently a normal domestic house converted for worship sometime between 233 and 256, when the town was abandoned after conquest by the Persians. It is less famous, smaller, and more modestly decorated than the nearby Dura Europos synagogue, though there are many other similarities between them.

The Dura-Europos house church with chapel area on right

New archaeological evidence is being discovered from time to time and on this note I would like to end this answer on the subject of St. Peter's Basilica. It is also has historical pats known as Old St. Peter's Basilica and The Basilica of Maxentius and Constantine

Now the original basilica was built over a Roman pagan necropolis known as the Vatican Necropolis. Catholic tradition has it that the Prince of the Apostles, St. Peter was martyred in the year 67 AD (64 AD by some) and was buried in this very necropolis. Thus Christians made use of this necropolis also.

There is some reference to St. Peter in Rome from the late 2nd century, in the Acts of Peter, even though the authenticity has been questioned by some.

The earliest reference to Saint Peter's death is in a letter of Clement, bishop of Rome, to the Corinthians (1 Clement, a.k.a. Letter to the Corinthians, written c. 96 AD). The historian Eusebius, a contemporary of Constantine, wrote that Peter "came to Rome, and was crucified with his head downwards," attributing this information to the much earlier theologian Origen, who died c. 254 AD.5 St. Peter's martyrdom is traditionally depicted in religious iconography as crucifixion with his head pointed downward.

Peter's place and manner of death are also mentioned by Tertullian (c. 160–220) in Scorpiace,6 where the death is said to take place during the Christian persecutions by Nero. Tacitus (56–117) describes the persecution of Christians in his Annals, though he does not specifically mention Peter. "They were torn by dogs and perished, or were nailed to crosses, or were doomed to the flames and burnt." Furthermore, Tertullian says these events took place in the imperial gardens near the Circus of Nero. No other area would have been available for public persecutions after the Great Fire of Rome destroyed the Circus Maximus and most of the rest of the city in the year 64 AD.

This account is supported by other sources. In The Passion of Peter and Paul, dating to the fifth century, the crucifixion of Peter is recounted. While the stories themselves are apocryphal, they were based on earlier material, helpful for topographical reasons. It reads, "Holy men ... took down his body secretly and put it under the terebinth tree near the Naumachia, in the place which is called the Vatican."12 The place called Naumachia would be an artificial lake within the Circus of Nero where naval battles were reenacted for an audience. The place called Vatican was at the time a hill next to the complex and also next to the Tiber River, featuring a cemetery of both Christian and pagan tombs. - Saint Peter's tomb

It is no coincidence that Constantine built the first St. Peter's Basilica in 330. Saint Peter's tomb is a site under St. Peter's Basilica that includes several graves and a structure said by Vatican authorities to have been built to memorialize the location of Saint Peter's grave. St. Peter's tomb is near the west end of a complex of mausoleums that date between about AD 130 and AD 300. The complex was partially torn down and filled with earth to provide a foundation for the building of the first St. Peter's Basilica during the reign of Constantine.

What might seem more than just a simple coincidence is that Our Lord said to St. Peter that he is the rock upon which He was to build His Church. When Pope Pius XI died (February 10, 1939), his successor Pope Pius XII ordered that a place be made available to put the body of his predecessor in the lower crypt area in the Vatican Basilica. In doing so workers discovered the ancient necropolis of Vatican Hill.

What they found during these excavations, which for many years (1940-1949) was done in almost total secret, partly because of the World War II, was utterly amazing.

During excavations under St Peter’s Basilica that began after the Second World War, archaeologists discovered a funerary monument with a casket built in honour of Peter and an engraving in Greek that read "Petros eni", or "Peter is here". - Bones attributed to St Peter found by chance in 1,000-year-old church in Rome

It is all too obvious by the graffiti found in the Vatican Necropolis that the tomb of St. Peter was a place of visitation after 67 AD.

A stone inscription in the church records the fact that it guards the relics of St Peter and other early popes, as well as martyrs Credit: Codice/Rai Uno

Time Magazine has an excellent article with many photos of the excavations of the Vatican Necropolis: LIFE at the Vatican: Unearthing History Beneath St. Peter's Basilica:

In December 1950 Pope Pius XII announced that bones discovered during the excavation could not conclusively be said to be Peter's. Two decades later, in 1968, Pope Paul VI announced that other bones unearthed beneath the basilica — discovered in a marble-lined repository, covered with a gold and purple cloth and belonging to a man around 5' 6" tall who had likely died between the ages of 65 and 70 — were, in the judgment of "the talented and prudent people" in charge of the dig, indeed St. Peter's.

For those interested, skeptics and believers alike, visitors are permitted into the necropolis under St. Peter's Basilica. Restrictions may apply to certain individuals or groups. Those who suffer specific and serious physical problems that could be effected by these conditions, including claustrophobia, should not visit. It is worth visiting in Rome, especially when one can see the Graffiti Wall and the place were the bones of St. Peter were discovered. "Upon this Rock I will build my Church!" I have been there and would recommend it to all.

Special visits to the necropolis underneath the Basilica, where the tomb of St. Peter is located, are only possible following special permission granted from time to time by the “Fabbrica di San Pietro”. Visits are organized according to the schedule set by the Excavations Office.

Visit the Tomb of Saint Peter and the Necropolis under the Vatican Basilica

Douglas Boin addresses the two main reasons given for the lack of archaeological evidence of Christianity during the period in question:

1. Theological: Christians applied a Biblical prohibition on idols to "Christian" objects
2. Economics: Christians were from the lowest class and couldn't afford to make these objects

In addition to evidence attributed to Christians given in other answers, there are two important items from non-Christians dated to the period in question.

First, if archeology is limited to items like art, crosses, or buildings, as Boin implies in his first post, the Alexamenos graffito found in Rome refutes this claim:

This "may be the earliest surviving depiction of Jesus and, if so, competes with an engraved gem as the earliest known pictorial representation of the Crucifixion of Jesus. It is hard to date, but has been estimated to have been made c. 200"¹ Unlike Christian art, the graffito must be given a higher degree of historical objectivity because it comes from a non-Christian who wrote, "Alexamenos worships [his] God." It proves the existence of Christians in Rome during the time in question and it shows the church in Rome found after the "period of obscurity" was worshiping the same God.

In his second post, Boin considers the people who make up the church; here he takes a broader view of what constitutes archaeological evidence and includes documents. His argument for an educated upper class is based on the obvious: Paul and others wrote. In an era of presumed illiteracy and a high cost of writing, Paul's letters demonstrate there were well-to-do Christians who had the resources to produce the type of artifacts which were found later.

I see two deficiencies in Boin's reasoning:

1. If documents are considered archaeological evidence (which of course they are), the claim none exists during the 150 years after 70 AD is patently false.
2. If Paul's letters are considered accurate descriptions of the make-up of the early church, these descriptions (not artifacts which are found hundreds of years later) should be considered as the standard for what is to follow.

The second significant piece of evidence from a non-Christian is a letter from Pliny the Younger to Trajan written around 112 AD:

It is my practice, my lord, to refer to you all matters concerning which I am in doubt. For who can better give guidance to my hesitation or inform my ignorance? I have never participated in trials of Christians. I therefore do not know what offenses it is the practice to punish or investigate, and to what extent. And I have been not a little hesitant as to whether there should be any distinction on account of age or no difference between the very young and the more mature; whether pardon is to be granted for repentance, or, if a man has once been a Christian, it does him no good to have ceased to be one; whether the name itself, even without offenses, or only the offenses associated with the name are to be punished.

Meanwhile, in the case of those who were denounced to me as Christians, I have observed the following procedure: I interrogated these as to whether they were Christians; those who confessed I interrogated a second and a third time, threatening them with punishment; those who persisted I ordered executed. For I had no doubt that, whatever the nature of their creed, stubbornness and inflexible obstinacy surely deserve to be punished. There were others possessed of the same folly; but because they were Roman citizens, I signed an order for them to be transferred to Rome.

Soon accusations spread, as usually happens, because of the proceedings going on, and several incidents occurred. An anonymous document was published containing the names of many persons. Those who denied that they were or had been Christians, when they invoked the gods in words dictated by me, offered prayer with incense and wine to your image, which I had ordered to be brought for this purpose together with statues of the gods, and moreover cursed Christ--none of which those who are really Christians, it is said, can be forced to do--these I thought should be discharged. Others named by the informer declared that they were Christians, but then denied it, asserting that they had been but had ceased to be, some three years before, others many years, some as much as twenty-five years. They all worshipped your image and the statues of the gods, and cursed Christ.

They asserted, however, that the sum and substance of their fault or error had been that they were accustomed to meet on a fixed day before dawn and sing responsively a hymn to Christ as to a god, and to bind themselves by oath, not to some crime, but not to commit fraud, theft, or adultery, not falsify their trust, nor to refuse to return a trust when called upon to do so. When this was over, it was their custom to depart and to assemble again to partake of food--but ordinary and innocent food. Even this, they affirmed, they had ceased to do after my edict by which, in accordance with your instructions, I had forbidden political associations. Accordingly, I judged it all the more necessary to find out what the truth was by torturing two female slaves who were called deaconesses. But I discovered nothing else but depraved, excessive superstition.

I therefore postponed the investigation and hastened to consult you. For the matter seemed to me to warrant consulting you, especially because of the number involved. For many persons of every age, every rank, and also of both sexes are and will be endangered. For the contagion of this superstition has spread not only to the cities but also to the villages and farms. But it seems possible to check and cure it. It is certainly quite clear that the temples, which had been almost deserted, have begun to be frequented, that the established religious rites, long neglected, are being resumed, and that from everywhere sacrificial animals are coming, for which until now very few purchasers could be found. Hence it is easy to imagine what a multitude of people can be reformed if an opportunity for repentance is afforded.

Pliny confirms the existence of Christians both in Bithynia et Pontus (his province) and in Rome. He records the behavior of these people who "gathered on a set day to sing responsively a hymn to Christ as to a god." This describes Christians who gathered to worship their God even though they lacked a building: exactly as Christians were doing before 70 AD (e.g. Acts 16:13).

Pliny states there were large number of Christians "of every age, every rank, and also of both sexes" who had "spread not only to the cities but also to the villages and farms." He implies the spread of Christianity was responsible for the decline of emperor worship in Pliny's province.²

Trajan replied and he too affirms the existence of Christians:

You observed proper procedure, my dear Pliny, in sifting the cases of those who had been denounced to you as Christians. For it is not possible to lay down any general rule to serve as a kind of fixed standard. They are not to be sought out; if they are denounced and proved guilty, they are to be punished, with this reservation, that whoever denies that he is a Christian and really proves it--that is, by worshiping our gods--even though he was under suspicion in the past, shall obtain pardon through repentance. But anonymously posted accusations ought to have no place in any prosecution. For this is both a dangerous kind of precedent and out of keeping with the spirit of our age.

Given the persecution of Christians and in particular Christian leaders³ during this period, the paucity of the type of evidence found later is hardly surprising. In fact, if this type of evidence did exist, Boin would rightly point to it as proof Roman persecution of Christians could not have happened. After all, how can a Christian be making crosses, manger scenes, and paintings, if these actions were certain to bring imprisonment and/or death?

Conclusion
First, as other answers show, there is evidence from this period. In particular there are two pieces from non-Christians which show Christians worshipped the crucified Christ as God.

Second, the question is anachronistic. It seeks the type of evidence found later in history rather than evidence expected from the previous period. Using Boin's approach, one should also conclude Christians did not exist before 70 AD because of the same lack of evidence. In other words, the lack of crosses, manger scenes, paintings, carvings, statutes did not begin after 70 AD, or any first century date: the "period of obscurity" began with the first Christians, who also never engaged in these actions.

Finally, if Paul's letters are considered accurate descriptions of the first Christians, one must ask what type of artifacts and "church services" are recorded? Here the record is clear: the first Christians focused solely on people, meeting with them in existing structures, like synagogues, public stadiums, and private houses.⁴

The development of the church in Corinth is a good illustration:

And he reasoned in the synagogue every Sabbath, and tried to persuade Jews and Greeks. When Silas and Timothy arrived from Macedonia, Paul was occupied with the word, testifying to the Jews that the Christ was Jesus. And when they opposed and reviled him, he shook out his garments and said to them, “Your blood be on your own heads! I am innocent. From now on I will go to the Gentiles.” And he left there and went to the house of a man named Titius Justus, a worshiper of God. His house was next door to the synagogue. (Acts 18:4-8 ESV)

When Paul left the synagogue, he went to someone's house. Paul never built a building, nor did he instruct others to build a "Christian synagogue." Paul never put up a sign or plaque to identify the church, nor did he give instructions for others to do so. There is no record of Paul making or using a cross, painting, or any artifact. The question is not whether they were prohibited: the issue is the first Christians didn't make or use them. Given this unambiguous documentary evidence of how the first Christians met and worshipped, it is unreasonable to claim the continuation of those practices is evidence Christianity did not continue uninterrupted after 70 AD.

The evidence from non-Christians show Christians between 70 AD and 220 AD were doing exactly the same things Christians did before 70 AD: gathering regularly to worship Jesus as God.

Notes:
1. Alexamenos graffito
2. The decline in emperor worship may be the motivating factor behind the letter. Pliny may be concerned Trajan has learned of the decline in sacrifices and empty temples. The letter is Pliny's "back-handed" attempt to show he has restored order. (Pliny is purposeful to point out his "test" is to have them worship Trajan's image.) The implied message is, "Emperor Trajan, I have put an end to this "heresy" and the people of my province are once again worshipping you as a god."
3. Christian martyrs
4. As also detailed in Pliny's letter.

Here are some standard evidences of the early existence of Christianity:

• Many extant Bible MSS date from before 350 AD such as p1 (3rd), p4 (3rd), p5 (3rd), p9 (3rd), p15 (3rd), p20 (3rd), p52 (about 125), p90 (2nd), p98 (2nd), p104 (2nd), and many more). There are also many important archaeological artifacts. (See a more complete list of MSS in UBS4, UBS5 NA27, NA28.)
• The voluminous writings of all the ante-Nicaean Fathers (e.g., Ignatius, Clement, Polycarp, Justin Martyr, Tertullian, and Origen), starting at about 110 AD. (My sets consist of The Apostolic Fathers by Holmes [Baker Academic] and Ante-Nicene Fathers [T&T Clark Edinburgh, Eerdmans Publishing])
• The writings of Josephus (about 90 AD) refer to Christians, Christian practice, Christian belief, and even Jesus.
• Chapel of Saint Ananias, Damascus, Syria, dates from the first century.
• A fragment of an inscription bearing the name of Pliny, Basilica of Sant'Ambrogio, Milan. Pliny the Younger, the Roman governor of Bithynia and Pontus (northern modern Turkey), wrote a letter to Emperor Trajan around 112 AD and asked for counsel on dealing with Christians. The letter (Epistulae X.96) details an account of how Pliny conducted trials of suspected Christians who appeared before him as a result of anonymous accusations, and in the letter he asks for the Emperor's guidance on how the the Christians should be treated.
• Tacitus (a Roman senator) wrote in his book (circa 116 AD) about Jesus and His execution under Pilate, and the existence of Christians in Rome ("Annals" book 15, chapter 44).
• The Roman historian Suetonius (c. 69 – after 122 CE) made references to early Christians and their leader in his work Lives of the Twelve Caesars (ca. 121 CE). The references appear in Claudius 25 and Nero 16, and they describe the lives of Roman Emperors Claudius and Nero.
• Documents excavated at Oxyrhynchus show many writings about Christianity dating from the first- and early second century.
• ΙΧΘΥΣ (Ichthys--fish inscriptions dating from the second century.
• Numerous Christian graves in the Catacombs of Rome dating from the second century.

There is much more, and all of it well known.

We should not be surprised we have as much as we do to document early Christianity. Up until Constantine issued the Edict of Toleration from Milan in 313, which legalized Christianity, Christianity was illegal and suffered numerous purges and persecutions, culminating with Diocletian's attempt to annihilate Christians. However, much evidence has survived, as I've documented above.

See also A History of Christianity Regions 2 & 4 (DVD) for more general information about early Christianity.