Who were the main Protestant precursors?

I believe the Holy Spirit combined with literacy followed by Biblical literacy would be the number one factor in the reformation. Too much power and corruption was held by the Catholic church primarily because people did not have access to scripture in their own language.

After years of much prayer, meditation, and struggle, Luther discovered the true meaning of God’s Word: “Then finally God had mercy on me, and I began to understand that the righteousness of God is a gift of God by which a righteous man lives, namely faith, and that sentence: The righteousness of God is revealed in the Gospel, is passive, indicating that the merciful God justifies us by faith, as it is written: ‘The righteous shall live by faith.’ Now I felt as though I had been reborn altogether and had entered Paradise. Luther's Romans Breakthrough

Luther's antisemitism: While many necessary things were reformed, not everything that needed to be reformed was reformed all at once.

Ironically, the ideology of Sola Scriptura mirrors the beliefs of Karaite Judaism, in spite of Martin Luther's antisemitism.

So technically, Karaite Judaism can be considered the "precursor" of Protestantism since the Hebrew Bible is still part of the Christian canon. According to Wikipedia:

Karaite Judaism (/ˈkɛərə.aɪt/) or Karaism (/ˈkɛərə.ɪzəm/; Hebrew: יהדות קראית, Modern: Yahadut Qara'it from, Tiberian: Qārāʾîm, meaning "Readers"; also spelt Qaraite Judaism or Qaraism)[a] is a Jewish religious movement characterized by the recognition of the written Torah alone as its supreme authority in halakha (Jewish religious law) and theology. Karaites maintain that all of the divine commandments handed down to Moses by God were recorded in the written Torah without additional Oral Law or explanation. It is distinct from mainstream Rabbinic Judaism, which considers the Oral Torah, as codified in the Talmud and subsequent works, to be authoritative interpretations of the Torah. As a result, Karaite Jews do not accept as binding the written collections of the oral tradition in the Midrash or Talmud.

When interpreting the Torah, Karaites strive to adhere to the plain or most obvious meaning (peshat) of the text; this is not necessarily the literal meaning, but rather the meaning that would have been naturally understood by the ancient Israelites when the books of the Torah were first written. By contrast, Rabbinic Judaism relies on the legal rulings of the Sanhedrin as they are codified in the Midrash, Talmud, and other sources to indicate the authentic meaning of the Torah.[b] Karaite Judaism holds every interpretation of the Torah to the same scrutiny regardless of its source, and teaches that it is the personal responsibility of every individual Jew to study the Torah, and ultimately decide personally its correct meaning. Karaites may consider arguments made in the Talmud and other works without exalting them above other viewpoints.

I recommend the book The Great Controversy if you are curious about the Christian reformation. It details nicely the early Protestant reformers, including reformations in England, Bohemia, Germany, Switzerland, France etc., and ties well into details on subsequent movements like Methodists, Millerites, and Seventh Day Adventists.

One of the earliest reformators were the Waldenses, there is an entire chapter about them:

The Waldenses were among the first of the peoples of Europe to obtain a translation of the Holy Scriptures. Hundreds of years before the Reformation they possessed the Bible in manuscript in their native tongue. They had the truth unadulterated, and this rendered them the special objects of hatred and persecution Pg 65... They were hunted to death; yet their blood watered the seed sown, and it failed not of yielding fruit. Thus the Waldenses witnessed for God centuries before the birth of Luther. Pg 78

In the Fourteenth centrury arose John Wyclife in England and the Lollards:

The “morning star of the Reformation.” John Wycliffe was the herald of reform, not for England alone, but for all Christendom. The great protest against Rome which it was permitted him to utter was never to be silenced Pg. 80 ... The papists had failed to work their will with Wycliffe during his life, and their hatred could not be satisfied while his body rested quietly in the grave. By the decree of the Council of Constance, more than forty years after his death his bones were exhumed and publicly burned, and the ashes were thrown into a neighboring brook. Pg 95.

Inspired by the writing of Wycliffe, reformers in Bohemia, John Huss (one of my favourite) and Jerome took their stand, and were both martyred:

At his [Jerome's] retraction, Jerome had assented to the justice of the sentence condemning Huss; he now declared his repentance and bore witness to the innocence and holiness of the martyr. “I knew him from his childhood,” he said. “He was a most excellent man, just and holy; he was condemned, notwithstanding his innocence.... I also—I am ready to die: I will not recoil before the torments that are prepared for me by my enemies and false witnesses, who will one day have to render an account of their impostures before the great God, whom nothing can deceive.”—Bonnechose, vol. 2, p. 151. Pg. 113

After which came Luther for Germany:

It was about this time that Luther, reading the works of Huss, found that the great truth of justification by faith, which he himself was seeking to uphold and teach, had been held by the Bohemian Reformer. “We have all,” said Luther, “Paul, Augustine, and myself, been Hussites without knowing it!” “God will surely visit it upon the world,” he continued, “that the truth was preached to it a century ago, and burned!”—Wylie, b. 6, ch. 1 GC 140.3 Pg 140

A contemporary of Luther, was Zwingli, a Swiss reformer:

“If Luther preaches Christ,” said the Swiss Reformer, “he does what I am doing. Those whom he has brought to Christ are more numerous than those whom I have led. But this matters not. I will bear no other name than that of Christ, whose soldier I am, and who alone is my Chief. Never has one single word been written by me to Luther, nor by Luther to me. And why? ... That it might be shown how much the Spirit of God is in unison with itself, since both of us, without any collusion, teach the doctrine of Christ with such uniformity.”—D'Aubigne, b. 8, ch. 9. Pg 174

And in France and Geneva, laboured John Calvin:

For nearly thirty years Calvin labored at Geneva, first to establish there a church adhering to the morality of the Bible, and then for the advancement of the Reformation throughout Europe. His course as a public leader was not faultless... But he was instrumental in promulgating truths that were of special importance in his time, in maintaining the principles of Protestantism against the fast-returning tide of popery, and in promoting in the reformed churches simplicity and purity of life, in place of the pride and corruption fostered under the Romish teaching. Pg 236

Tyndale then carried the flames ignited by Wyclife and Luther back to England. Tyndale was martyred for distributing to the people... the English Bible:

Tyndale responded: “Do you know who taught the eagles to find their prey? Well, that same God teaches His hungry children to find their Father in His word. Far from having given us the Scriptures, it is you who have hidden them from us; it is you who burn those who teach them, and if you could, you would burn the Scriptures themselves.”—D'Aubigne, History of the Reformation of the Sixteenth Century, b. 18, ch. 4. Pg 245

Conclusion

Therefore, we owe a great deal to all these reformators and more. Each engaged in the work at great peril to their lives. However, coming from such a long period of scriptural and spiritual darkness, it was not possible for one man to understand all doctrines perfectly. God allowed great light to shine on the minds of chosen men, but He led them step by step, as they could bear it. I leave you with a quotation of from John Robinson, a Puritan, spoken to the pilgrims as they departed for the new world... America:

The Lutherans cannot be drawn to go beyond what Luther saw; ... and the Calvinists, you see, stick fast where they were left by that great man of God, who yet saw not all things. This is a misery much to be lamented; for though they were burning and shining lights in their time, yet they penetrated not into the whole counsel of God, but were they now living, would be as willing to embrace further light as that which they first received.”—D. Neal, History of the Puritans 1:269. Pg. 292

Martin Luther is not considered as being responsible for the entire Protestant reformation, although his contribution had a huge impact on the events that followed. One of the difficulties in doing justice to this question is the huge amount of historical data available, both pre- and post-Luther. What follows is simply a brief summary of key events and persons, from a Protestant perspective.

During the 13th century, long before Martin Luther nailed his 95 theses onto the door of Wittenberg church in 1517, a group of Roman Catholics ran into trouble after they started handing out Bibles that were not in Latin, and began to preach without permission. They were followers of Peter Waldo, a wealthy merchant of Lyons, and they became known as Waldensians.

The Waldensians’ back-to-the-Bible approach appealed to many, and the movement quickly spread rapidly to Spain, northern France, Flanders, Germany, southern Italy, and even Poland and Hungary. But the Catholic Church did not take kindly to the Waldensian call to reform. In 1181 the archbishop of Lyons excommunicated the Waldensians. Three years later, the pope declared them to be heretics. In 1215 the Fourth Lateran Council declared an anathema on Waldensian doctrine. In the 1230s, persecution against the Waldensians increased and lasted for three hundred years. Eventually, most Waldensians became part of the churches of the Reformation, such as Presbyterian, Lutheran, or Reformed. But today there are still Waldensian churches in existence in Germany, Italy, Uruguay, Argentina, the United States, and elsewhere. Source: https://www.gotquestions.org/Waldensians.html

Moving on the the 14th century, we come to John Wycliffe (1330-1384) who became known as the Morningstar of the Reformation.

Around 1370 John Wyclliffe began writing some controversial material. He wrote about the roles of government and church authorities in 1370, arguing that the ungodly have no right to rule... Pope Gregory XI condemned 18 of Wycliffe’s statements in 1377, calling Wycliffe “The Master of Errors,” and in 1378 Wycliffe was forced to retire from public life. After the Peasants’ Revolt in which Wycliffe’s disciples were implicated, Wycliffe withdrew to Lutterworth (in England) and continued writing until his death in 1384. Source: https://www.gotquestions.org/John-Wycliffe.html

His followers were known as the Lollards:

The Lollard doctrine, or Lollardism, is most clearly spelled out in a 1395 document called Twelve Conclusions. In addition to the Twelve Conclusions, the Lollards also believed that the primary duty of the priests should be to preach and that every person should have access to the Bible in his own language. Wycliffe and the Lollards were responsible for an English translation of the Bible. In 1401 the Lollard Henry Sawtrey was burned for his faith. In 1414 the Lollards were involved in a military revolt led by Sir John Oldcastle and defeated by Henry V. Fearing reprisals, the remaining Lollards were driven into hiding. This actually facilitated the spread of their teaching and influence. Their influence spread as far as Czechoslovakia and Jan Hus who in turn influenced Martin Luther. Source: https://www.gotquestions.org/Lollards.html

We now come to the late 14th – early 15th century and Jan Hus (1369–1415), a Roman Catholic priest in Bohemia (located in modern-day Czech Republic) who became a pre-Protestant Reformation reformer of the church and who was burned at the stake for his opposition to the Church of Rome.

Reading the writings of John Wycliffe further influenced Jan Hus in an anti-Catholic direction. The followers of Jan Hus, known as Hussites, continued, expanded, and intensified the rebellion against the Roman Catholic Church. The popes pronounced a series of crusades against the Hussites, which became known as the Hussite Wars. Each of the four crusades between 1419 and 1434 met with defeat at the hands of the Hussites. Within 100 years, nearly 90 percent of Bohemians were Hussite Christians. Source: https://www.gotquestions.org/Jan-Hus.html

Post-Luther came other reformers:

During the 16th century, other godly men stood in opposition to the Church of Rome—Jakob Hutter (founder of the Hutterites), John Knox of Scotland, William Tyndale (martyred for translating the New Testament into English), John Calvin of France, Ulrich Zwingli of Switzerland, and the English reformers Cranmer, Latimer, and Ridley (all burned at the stake). Source: https://www.gotquestions.org/middle-ages.html

The Lutheran Church is most closely identified with the teachings of Martin Luther and he could be identified as its founder. However, the majority of Reformed Protestant denominations are far removed from some aspects of Lutheran theology, and would denounce any anti-Semitism.

Who were the main Protestant precursors?

Martin Luther certainly had precursors in regard to the Reformation. While Luther set the reformation into motion in 1517, others were also involved in the process.

All in all, I believe what you are looking for is to be more informed about Proto-Protestant leaders and preachers.

Proto-Protestantism, also called pre-Protestantism or pre-Reformation movements, refers to individuals and movements that propagated ideas similar to Protestantism before 1517, which is usually considered the starting year for the Reformation era. Major representatives were Peter Waldo (c. 1140 – c. 1205), John Wycliffe (1320s–1384), Jan Hus (c.  1369 – 1415) and the movements they started.

Peter Waldo and the Waldensians

In the early 1170s, Peter Waldo founded the Waldensians. He preached for strict adherence to the Bible, for simplicity and poverty, against Catholic dogmas, like the purgatory and transubstantiation which led to conflicts with the Roman Catholic Church. He initiated, and contributed to, a translation of the New Testament into the vernacular, the Arpitan (Franco-Provençal) language.

The Waldensians had adopted ideas that in the late 1130s, Arnold of Brescia, an Italian canon regular, had developed in a first attempt to reform the Roman Catholic Church. His teachings on apostolic poverty gained currency among Arnoldists. By 1215, the Waldensians were declared heretical and subject to persecution.

John Wycliffe and the Lollards

John Wycliffe (1320s–1384) was an English theologian and professor at the University of Oxford who developed many ideas similar to those later promoted in the Reformation. He rejected papal authority over secular power, translated the Bible into vernacular English, and preached anticlerical and biblically-centred reforms. Wycliffe's teachings were spread by his followers, known as Lollards.

Jan Hus and the Hussites

Beginning in the first decade of the 15th century, Jan Hus, a Czech Catholic priest and professor who was influenced by John Wycliffe's writings, founded the Hussite movement. He was burned at the stake as a heretic in 1415. After his execution, a revolt erupted. Hussites defeated five continuous crusades proclaimed against them by the Pope.

Later on, theological disputes caused a split within the Hussite movement. Utraquists maintained that both the bread and the wine should be administered to the people during the Eucharist. Another major faction were the Taborites, who opposed the Utraquists in the Battle of Lipany during the Hussite Wars. There were two separate parties among the Hussites: moderate and radical movements. Other smaller regional Hussite branches in Bohemia included Adamites, Orebites, Orphans and Praguers.

Less influential early reformers

Throughout the Middle Ages, there were many Christian sects, cults and movements whose teachings foreshadowed later Protestant movements. Some of the main groups were:

Paulicians – an Armenian group (6th to 9th centuries) who sought a return to the purity of the church at the time of Paul the Apostle.

Tondrakians - an Armenian group (9th to 11th centuries) who advocated the abolition of the Church along with all its traditional rites.

Bogomils - a group arising in the 10th century in Bulgaria, Macedonia and the Balkans who sought a return to the spirituality of the early Christians and opposed established forms of government and church.

Arnoldists - a 12th-century group from Lombardy who criticized the wealth of the Catholic Church and preached against baptism and the Eucharist.

Petrobrusians were 12th-century followers of Peter of Bruys in southeastern France who rejected the authority of the Church Fathers and of the Catholic Church, opposing clerical celibacy, infant baptism, prayers for the dead and organ music.

Henricans were 12th-century followers of Henry of Lausanne in France. They rejected the doctrinal and disciplinary authority of the Church, did not recognize any form of worship or liturgy and denied the sacraments.

Brethren of the Free Spirit – a term applied in the 13th century to those, primarily in the Low Countries, Germany, France, Bohemia and northern Italy, who believed that the sacraments were unnecessary for salvation, that the soul could be perfected through imitating the life of Christ, and that the perfected soul was free of sin and beyond all ecclesiastical, moral and secular law.

Apostolic Brethren (later known as Dulcinians) – a 13th- to 14th-century sect from northern Italy founded by Gerard Segarelli and continued by Fra Dolcino of Novara. The Apostolic Brethren rejected the worldliness of the church and sought a life of perfect sanctity, in complete poverty, with no fixed domicile, no care for the morrow, and no vows.

Neo-Adamites – a term applied in the 13th to 15th centuries to those, including Taborites, Picards and some Beghards, who wished to return to the purity of the life of Adam by living communally, practicing social and religious nudity, embracing free love and rejecting marriage, and rejecting individual ownership of property.

None of these persons or groups seem to be anti-Semitic as Martin Luther was.

As for Luther’s views on Judaism, he seems to be influenced by Anton Margaritha.

Luther's main works on the Jews were his 65,000-word treatise Von den Juden und Ihren Lügen (On the Jews and Their Lies) and Vom Schem Hamphoras und vom Geschlecht Christi (Of the Unknowable Name and the Generations of Christ) — reprinted five times within his lifetime — both written in 1543, three years before his death. It is believed that Luther was influenced by Anton Margaritha book Der gantze Jüdisch Glaub (The Whole Jewish Belief). Margaritha, a convert to Christianity who had become a Lutheran, published his antisemitic book in 1530 which was read by Luther in 1539. In 1539, Luther got his hands on the book and immediately fell in love with it. “The materials provided in this book confirmed for Luther that the Jews in their blindness wanted nothing to do with faith and justification through faith.” Margaritha's book was decisively discredited by Josel of Rosheim in a public debate in 1530 before Charles V and his court, resulting in Margaritha's expulsion from the Empire. - Martin Luther and antisemitism

No, Luther certainly should not be viewed as "responsible [for] the entire protestant reformation". Throughout previous centuries, various non-Catholic church groups held to some beliefs that came to be recognisable as Reformation theology and practice. However, that subject is so vast, it cannot possibly be covered here. I recommend you study the book "The Pilgrim Church" by E.H. Broadbent (Pickering Classic first published in 1931 through to 1985 at least.)

Prior to the reformation starting, Catholic scholar Erasmus (1467-1536) was a serious editor of Latin and Greek texts and produced an epoch-making edition of the Greek New Testament (the first ever printed and published, in 1516, the year before the Reformation began). The Romanists furiously blamed him for ‘letting Luther happen’. He was spoken of obliquely as "The egg that Luther hatched".

In 1505 Erasmus had published Valla’s work as ‘Adnotationes in Novum Testamentum’. Valla had earlier incurred displeasure in Rome by showing to be a mere fable the supposed MSS relating to a ‘Donation of Constantine’ bestowing temporal power with the Bishops of Rome. Valla declared that theologians must work from the grammatical sense for the meaning of the text, to which Erasmus agreed.

Erasmus next travelled a lot, searching for Greek NT MSS, and then managed to construct a single continuous text of the Greek NT through comparisons and collation of a few MSS. His 1516 edition had his version of the Latin Vulgate NT, supported in a parallel column by his newly compiled text of the Greek NT. He used the Greek as a plumb line to vindicate his Latin differences. Many valid criticisms of his work were admitted to by Erasmus himself from the start. A fourth, improved edition was later produced.

Before Luther got going, Erasmus had produced his NT in its original Greek. Luther got the printed text in Wittenberg as he was lecturing on the 9th chapter of Romans. From Erasmus's accompanying translation he learned of the inaccuracy of the Vulgate rendering of 'do penance' instead of 'be penitent'.

Erasmus and Luther had much in common. Both insisted that the church of their day had relapsed into a Judaistic type of legalism condemned by the apostle Paul. Christianity, said Erasmus, has been made to consist not in loving one's neighbour but in abstaining from butter and cheese during Lent. What good are indulgences to those who do not mend their ways? Both men had a quarrel with the pope, Luther because the pontiffs imperilled the salvation of souls, Erasmus because they fostered external ceremonies and, at times, impeded free investigation.

Poor Erasmus ended up falling between two stools – the Romanists blamed him furiously for ‘letting Luther happen’. He was spoken of obliquely as "The egg that Luther hatched". But the Lutherans could not accept his not falling in with Luther. Article by C.P. Hallihan in Issue No. 615, April-June 2016, Quarterly Record (Trinitarian Bible Society)

My material is a combination from that article and the book "Here I Stand - Martin Luther" by Roland Bainton (Lion Publishing 1978). As for your second question about Luther's antisemitism, that is also admitted to in Bainton's book.

"He had earlier believed that contemporary Jews could not be blamed for the sins of their fathers, and sympathised with why they hated Catholicism so much; “We should use toward the Jews not the pope’s but Christ’s law of love.” Sadly, he came to change that view when his endeavour to proselytize some rabbis failed. A rumor that a Jew had been suborned by the papists to murder him was believed. Later, news came that in Moravia, Christians were being induced to Judaize. Then he came out with a vulgar blast in which he recommended that all Jews be deported to Palestine. Failing that, they should be forbidden to practice usury, should be compelled to earn their living on the land, their synagogues should be burned, and their books (including the Hebrew scriptures) should be taken from them. One paragraph reads, "Therefore be on your guard against the Jews, knowing that wherever they have their synagogues, nothing is found but a den of devils in which sheer self¬glory, conceit, lies, blasphemy, and defaming of God and men are practiced most maliciously and veheming his eyes on them."

That tract is a shame and no excuse is offered. Quote from p379-380 of Bainton's book. Many Protestant groups today would offer no excuse for Luther's antisemitism either. We are far removed from Luther's era and popular attitudes in Europe back then. I hope this goes some way to answering your questions, the scope of which is vast.