To understand the question I have read chapter 7 from http://www.judaismvschristianity.com/ and here is my impression of the argument as someone who has spent thirty years studying the scriptures, church history and doctrine.

First, the article is not remotely scholarly nor is there any scholarly debate to support it. Outside of a few confused interpretations of a few biblical events, it does not cite any external history or facts to support the argument. For this reason there is no 'scholarly refutation' of the argument, since the argument is not recognized academically. Therefore, my response may be the closest thing to scholarly as I am at least willing to investigate the claim and provide a reasoned response to it, whereas most scholars would not be bothered.

Second the article uses information that naturally oppose the argument itself making it not only not scholarly but completely illogical.

As the argumentation is so flimsy and unsupported by scripture and church history I will only outline the authors primary argument and show how easily it can be dismissed as sham logic.

It is argued that his Apostleship was unrecognized by the other Apostles.

Answer: Anyone familiar with scripture would realize that he was fully recognized by all the Apostles, otherwise the letters of the other Apostles would have warned congregations of this 'false Apostle'. Instead they gave him full support. For example, Peter knowing Paul often referred to himself as 'an Apostle' in his letters (because there were false teachers who challenged his Apostleship) said this:

Bear in mind that our Lord’s patience means salvation, just as our dear brother Paul also wrote you with the wisdom that God gave him. He writes the same way in all his letters, speaking in them of these matters. His letters contain some things that are hard to understand, which ignorant and unstable people distort, as they do the other Scriptures, to their own destruction. Therefore, dear friends, since you have been forewarned, be on your guard so that you may not be carried away by the error of the lawless and fall from your secure position. (2 Peter 3:15-17, NIV)

Here Peter infers Paul's letters are also the scriptures which unstable people distort. That means when Paul said he was an Apostle, Peter included the statement as scripture.

This verse also, so succinctly describes the argument of this author in his article on the website. Everything argued in so contrary to serious thought and expresses such ignorance of the scripture that it must come from an 'unstable mind'. Even a stable minded unbeliever would recognize that if Paul had hijacked Christianity history would have recorded great conflict between Peter and Paul, yet no such conflict is recorded in Christian history. Furthermore, a stable mind that does believe in Christ would find it highly improbable that the Messiah would have established a new kingdom and glorious phase of Salvation history upon a Rock (which the gates of hell could not prevail against) and then immediately allow a false Apostle, quickly overthrowing that kingdom. (Ref Math 16:18)

The rest of his arguments are similar and as easy to shred into pieces, but since the foundation of his theories are already laid waste there is no apparent purpose to proceed.

I have long suspected Paul was a false apostle. Many in the past have, and many today still do, believe this to be the case.

When we read the epistles attributed to Paul or even the book of Acts many red flags seem to appear. For starters, Paul began by arresting/persecuting Christians. In the book of Acts we are told Paul was present at the murder of a Stephen who was a known Christian. Acts goes on to tell us Paul experienced a conversion on his way to Damascus to further persecute Christians. This account is mentioned in 9:1-31, 22:1-22, and 26:9-24. This whole story is suspicious for a number of reasons. First, the way Jesus appears to Paul in the story is not biblical. Jesus always appeared in the flesh, not as a light as specified in Paul's supposed account. Second, the account given is contradictory. Acts 9:7 tells us the individuals traveling with Paul heard Jesus' voice but saw no one. Acts 22:9 tells us instead the individuals with Paul saw a light but did not hear Jesus' voice. Third, is the improbable claim that Paul was on his way to Damascus to round up Christians. This is very unlikely considering Damsacus was a predominantly pagan territory at the time under Nabataean control. Paul would have had no authority to round any one up. Fourth, where are those individuals that were with Paul to verify this incident? It surely would make it more believeable if there were witnesses.

In the New Testament Paul is referred to as an apostle 22 times. Only twice is this done by someone other than Paul himself. It is not from Jesus, or any of the original apostles(disciples), but from Paul's friend and personal press secretary Luke. It is obvious from reading the accounts of Paul, Acts for instance, that the disciples did not believe him to be one of them. There was obviously no love loss between the disciples and Paul.

Another area of contradiction is in Paul's teachings. Paul was a stout supporter of faith. This in itself is not a bad thing. The problem is Paul emphasized faith over everything else; including action. Paul's teachings on salvation apart from works contradicts other clear teachings in the New Testament such as Matthew 5:19, 25:31-46, John 15:10, James 1:22-25, 2:24. Other than Paul, the New Testament teachings are predominantly works based. Another area of difference in Paul's teachings and the rest of the New Testament are his views in the Jewish Law. Paul's letters contain a substantial amount of criticism of the Law. This view would not have held well with the Jewish Jesus and his Jewish disciples.

Many people over the centuries have doubted Paul. Many considered themselves Christian (followers of Jesus).

Ebionites (possibly the first Christians) believed him to be a false apostle.

These men (Ebionites), moreover, thought that it was necessary to reject all the epistles of the apostle [Paul], whom they called an apostate from the law
— Eusebius 325 AD

Some significant early church fathers had issues with Paul.

Macarius Magnes (early church theologian) found Paul to be contradictory.

[Paul] says, ‘As many as are under the Law are under a curse’ (Gal 3:10). The man who writes to the Romans, ‘The Law is spiritual’ (7:14), and again, ‘The Law is holy and the commandment holy and just’ (7:12), places under a curse those who obey that which is holy!... In his Epistles … he praises virginity (I-Tim 4:1, I-Cor 7:25), and then turns round and writes, ‘In the latter times some shall depart from the faith,... forbidding to marry’ (I-Tim 4:1-3).... And in the Epistle to the Corinthians he says, ‘But concerning virgins I have no commandment of the Lord’
— (I-Cor 7:25). ca. 300

Augustine (early church theologian) accused Paul of being a liar.

If it be possible for men to say and believe that, after introducing his narrative with these words, ‘The things which I write unto you, behold, before God, I lie not’, the apostle (Paul) lied when he said of Peter and Barnabas, ‘I saw that they walked not uprightly, according to the truth of the gospel’,... [then] if they did walk uprightly, Paul wrote what was false; and if he wrote what was false here, when did he say what was true?
— 397 AD

Jerome (early church theologian) accused Paul of hypocrisy.

Porphyry ... accuses Paul of presumption because he dared to reprove Peter and rebuke him to his face, and by reasoning convict him of having done wrong; that is to say, of being in the very fault which he himself, who blamed another for transgressing, had committed.... Oh blessed Apostle Paul— who had rebuked Peter for hypocrisy, because he withdrew himself from the Gentiles through fear of the Jews who came from James—why are you, notwithstanding your own doctrine, compelled to circumcise Timothy (Acts 16:3), the son of a Gentile, nay more, a Gentile himself?
— 404 AD

Jerome's continued criticism of Paul.

Paul does not know how to develop a hyperbaton [i.e., a change of normal word order for emphasis], nor to conclude a sentence; and having to do with rude people, he has employed the conceptions, which, if, at the outset, he had not taken care to announce as spoken after the manner of men, would have shocked men of good sense.
— 411 AD

Many modern Christian theologians have found reason to doubt Paul.

In the teachings of Christ, religion is completely present tense: Jesus is the prototype and our task is to imitate him, become a disciple. But then through Paul came a basic alteration. Paul draws attention away from imitating Christ and fixes attention on the death of Christ The Atoner. What Martin Luther, in his reformation, failed to realize is that even before Catholicism, Christianity had become degenerate at the hands of Paul. Paul made "Christianity the religion of Paul, not of Christ. Paul threw the Christianity of Christ away, completely turning it upside down, making it just the opposite of the original proclamation of Christ.
— theologian Soren Kierkegaard

True Christianity, which will last forever, comes from the gospel words of Christ not from the epistles of Paul. The writings of Paul have been a danger and a hidden rock, the causes of the principal defects of Christian theology. — theologian Ernest Renan

Paul himself stands in the twilight zone of heresy. In reading Paul, one immediately encounters a major difficulty. Whatever Jesus had preached did not become the content of the missionary proclamation of Paul. . . . Sayings of Jesus do not play a role in Paul 's understanding of the event of salvation. . . . Paul did not care at all what Jesus had said. . . . Had Paul been completely successful very little of the sayings of Jesus would have survived.
— theologian Helmut Koester

What kind of authority can there be for an 'apostle' who, unlike the other apostles, had never been prepared for the apostolic office in Jesus' own school but had only later dared to claim the apostolic office on the basis on his own authority? The only question comes to be how the apostle Paul appears in his Epistles to be so indifferent to the historical facts of the life of Jesus. . . . He bears himself but little like a disciple who has received the doctrines and the principles which he preaches from the Master whose name he bears.
— theologian Ferdinand Christian Baur

Paul. . . . did not desire to know Christ. . . . Paul shows us with what complete indifference the earthly life of Jesus was regarded. . . . What is the significance for our faith and for our religious life, the fact that the Gospel of Paul is different from the Gospel of Jesus?. . . . The attitude which Paul himself takes up towards the Gospel of Jesus is that he does not repeat it in the words of Jesus, and does not appeal to its authority. . . . The fateful thing is that the Greek, the Catholic, and the Protestant theologies all contain the Gospel of Paul in a form which does not continue the Gospel of Jesus, but displaces it.
— theologian Albert Schweitzer

It is most obvious that Paul does not appeal to the words of the Lord in support of his. . . . views. when the essentially Pauline conceptions are considered, it is clear that Paul is not dependent on Jesus. Jesus' teaching is -- to all intents and purposes -- irrelevant for Paul."
— theologian Rudolf Bultmann

If one may be allowed to speak rather pointedly the Apostle Paul was the only Arch-Heretic known to the apostolic age.
— theologian Walter Bauer

As I stated earlier, many today as well as many in the past, who doubt Paul's credibility consider themselves Christian. They have come to realize much aspects of Paul are anti-Jesus. This has led them to either become weary of Paul or just out right reject him. They believe a person can be a follower of Jesus without being a believer of Paul. I agree with them whole heartedly.

Let me conclude with the words of the author of the Declaration of Independence and the 3rd President of the United States.

Paul was the great Coryphaeus, and first corrupter of the doctrines of Jesus.
— Thomas Jefferson

The criticisms that Paul 'invented' Christianity, or 'distorted' the message of Jesus, often boil down to two primary claims:

1. Paul sought to abolish Torah observance
2. Paul deified Jesus, equating him with the God of Israel

1. Paul, and the role of Torah

A recent publication, The Jewish Annotated New Testament, contains commentary and perspectives by leading Jewish scholars. In the back is an essay titled Paul in Jewish Thought by Daniel R. Langton.¹

In the opening of this essay, Langton summarizes a common Jewish view on Paul:

Generally speaking, Jews have regarded the apostle to the Gentiles suspiciously as a kind of self-hating Jew and as the "real" founder of the Christian religion.

Langton surveys a few Jewish perspectives on Paul from throughout history. In early Rabbinic literature, Langton sees 'some tantalizing references' to Paul as an antinomian who rejected the instruction of his teacher Gamaliel. The Medieval Toledot Yeshu ('Story of Jesus') apparently conflates Peter and Paul into a single individual: 'Simeon Kepha (or Paul, as he was known to the Nazarenes)'. This 'Paul' is again accused of rejecting Torah and introducing completely new practices into the community of 'those who followed Yeshu as the Messiah'.

The general portrait is that Paul brought in such innovation, the Christian movement was no longer recognizable as what had come before him. Even still, Langton insists, 'such an image was by no means widespread'. But then Langton comes to the nineteenth century rise of critical scholarship:²

German Protestant biblical criticism increasingly viewed Christianity as brought about by Paul's universalistic teaching. As the cliché states, Paul turned the religion of Jesus into the religion about Jesus. Among the earliest proponents of this view was the German scholar Heinrich Graetz (1817-91), whose immensely influential History of the Jews (1853-76 [English translation 1891-98]) presented Paul as the "inventor" of Christianity, distinguished Paul's superficial Jewish learning from Jesus' high-mindedness and moral purity, and argued that Paul's antinomian theology made him the "destroyer of Judaism."

In other words, the specific idea that Paul created a new religion by distorting Jesus' message mostly originated in the nineteenth century. Langton highlights other scholars who claimed Paul 'injected pagan elements into the religion of Jesus', including:

• American Reform rabbi, Kaufmann Kohler (1843-1926), specifically his Jewish Encyclopedia article 'Saul of Tarsus'
• German philosopher, Martin Buber (1878-1965)
• Chief Rabbi of the United Synagogue in the United Kingdom, Jonathan Sacks (1948-), specifically his One People?: Tradition, Modernity, and Jewish Unity

But in contrast to Paul's critics, Langton also lists a few who 'argued that Paul remained authentically Jewish':

• German Reform rabbi, Leo Baeck (1873-1956), specifically his article 'The Faith of Paul'
• Montefiore
• Hans-Joachim Schoeps
• Samson Raphael Hirsch
• Joseph Krauskopf
• Isaac Mayer Wise

Close to the end of his essay, Langton summarizes the views of a few more recent Jewish commentators on Paul:³

Mark Nanos (1954-) has gone so far as to claim that Paul was entirely Torah-observant and that he expected other Jewish followers of Jesus to be so as well. . . . Daniel Boyarin (1946-) maintains that despite the fact that Paul had found the Law problematic, his letters ("the spiritual autobiography of a first-century Jew") show him to be a Jew facing many of the same kinds of challenges that Jews face today . . .

Mark Nanos also has an essay included in The Jewish Annotated New Testament, titled 'Paul and Judaism'. In this essay, Nanos writes:⁴

More controversial [than accepting Paul 'was born and raised a Jew'] is whether he continued to practice Judaism after his change from being a persecutor of the followers of Jesus to becoming an apostle to the nations (i.e., the Gentiles).

Nanos explains Paul with this:⁵

Paul's letters arguably indicate that he lived in a Torah-observant manner, including eating according to prevailing halakhic conventions for an observant Jew. But that is not the way he has been most commonly interpreted. Rather, his urging of non-Jews to remain non-Jews, and his own self-deprecating comments about the supposed superiority of his standing as a Jew relative to their own questionable standing in the larger Jewish communities, has led many to suppose that Paul was demeaning the value of Jewish identity and behavior, of Torah as well as Israel. If, however, we look at the rhetorical context of his comments, we see that in all his extant letters they are directed to non-Jews.

Bringing this into a context of the compatibility of Paul and Jesus: Paul himself did not stop following the Torah, and did not expect Jewish followers of Jesus to stop either. He only argued that non-Jews were not required to start following the Torah after coming to believe in Jesus as the messiah, and this argument spilled over into his letters.

Because Jesus said virtually nothing (recorded) on the issue of non-Jews following Torah (and there were a variety of Jewish views on this subject at the time, as there are now), and because Paul did not expect Jewish followers of Jesus to reject Torah (according to Langton, Nanos, etc.), there is little ground to see Jesus' and Paul's teachings as explicitly contradictory on this matter.

2. Paul, and Jesus as God

The other question is whether Paul was the first to deify Jesus, identifying him with the God of Israel. There is huge disagreement on this point in the scholarly community. Some take the critical opinion that Paul perverted Jesus' message by deifying him. Others argue that the perception of Jesus as God is entirely compatible with Jewish thought. Still others claim that neither Paul nor any of the earliest Christians actually perceived Jesus as God.

2a. Christians before Paul identified Jesus as God

Arguably, Richard Bauckham and Larry Hurtado are the two most well-known scholars who argue that not only did the earliest followers of Jesus (i.e. before Paul) perceive Jesus to be 'God', this idea, while never-before-seen in Judaism, was completely at home within the wider stream of Jewish thought, and should not be understood as a pagan 'corruption'.

In summary, Bauckham and Hurtado make similar arguments in their writings⁶ that, prior to Jesus, different Jewish groups wrote about exalted humans (e.g. Enoch, Abraham, or Moses), specialized angels (e.g. Michael, or Yahoel), or divine hypostases (e.g. God's Wisdom, or God's Logos) who mediated between God and his creation in a unique way. The language, ideas, and tropes surrounding these figures were then applied by the earliest Christians to Jesus, with one unique 'mutation': Jesus' followers identified him with the God of Israel:⁷

Now I turn to my second objective, namely, to demonstrate that earliest Christian devotion [to Jesus] constituted a significant mutation or innovation in Jewish monotheistic tradition. By "mutation" I mean that earliest Christian devotion was a direct outgrowth from, and indeed a variety of, the ancient Jewish tradition.

Please note that our knowledge of such mediator figures largely comes from the Dead Sea Scrolls, a huge swath of Jewish literature that was not discovered until long after Paul's critics had been accusing him of creating a new religion in his deification of Jesus. The people who accused Paul of corrupting Judaism into something pagan that allowed for a divine Jesus had no idea such language, ideas, and tropes were actually very common place within the Judaism of Jesus' and Paul's time, even if the actual perception of a human as God was a distinct 'innovation' within 'the Jesus movement'.

Hurtado argues that this identification of Jesus with God is evident in hymns (e.g. Philippians 2.6-11; Colossians 1.15-20) and creeds (1 Corinthians 8.6; perhaps also the Maranatha, 'Come, Lord!' of 1 Corinthians 16.22) that existed before Paul embedded them into his own letters, as well as devotional practices that had been going on before Paul began following Jesus (e.g. prayer and worship for God are directed 'through' Jesus; baptism is done in Jesus' name; Jesus is the center of the Eucharist).

In other words:⁸

All along, the evidence in the Pauline letters pointed to an origin of the cultic veneration of Jesus [i.e. as God] in the earliest years of the Christian movement and among Christians from an undeniably Jewish background, including Aramaic-speaking believers. Some earlier history of religion researchers apparently found it difficult to reconcile this with their notions of what could be expected of Jewish Christians. Thus they attributed the origin of the cultic veneration of the risen Christ to a later stage of the Christian movement and invoked the influence of pagan cults as the cause.

2b. Paul and early Christians did not identify Jesus as God

Still, an opposing view is that Paul never deified Jesus, nor did any of the other early Christians, and that insisting as much is a severe misinterpretation of Paul's letters.

Pamela Eisenbaum, a Jewish scholar of Paul, insists he remained 'a radical Jewish monotheist' even after becoming a follower of Jesus. Paul certainly revered Jesus greatly, above and beyond the Torah and other people. For Paul, 'Jesus has replaced Torah as the key to salvation'.⁹ But according to Eisenbaum, Paul made a consistent distinction between Jesus and God in his theology, even when writing his letters:¹⁰

. . . from the perspective of outsiders to the Christian tradition, Paul has sometimes been ridiculed for having abandoned monotheism. Such ridicule is part of a more general theological critique, advanced for centuries by Muslims and Jews, against the Christian doctrine of the incarnation, namely that God became human, and the notion of a triune God, namely that God is three-in-one, Father, Son, and Holy Ghost. . . . Jesus is clearly a divine figure of unique status in Paul's letters, and this has led many historians to conclude that devotion to Christ as developed by Paul must have come from outside—that is, non-Jewish—influences.

Eisenbaum interacts with Hurtado on his views on the early acceptance of Jesus as a 'divine figure of unique status', but she homes in on different key points of the ancient Jewish texts about mediator figures and how those same ideas are used for Jesus. Her conclusion is that:¹¹

A shift in devotional focus from God to Christ may have already begun with other New Testament writers, but these writers come at least a generation after Paul. As noted by many scholars, Paul carefully distinguished between Jesus and God and did not worship Jesus as if he were a god, nor does the apostle treat Christ as the equivalent of God, the use of similar language notwithstanding. Rather, confessing Jesus as Lord was supposed to point people toward God; it was not meant to distract people from God nor to complicate the unitary nature of God. One misunderstands Paul if one misses this point.

Conclusion

While Paul's exact views on either Torah or Jesus may still be debated, there is sufficient reason for thinking Paul did not 'invent' Christianity. Whatever his views on Torah or Jesus, there is at least a historical congruency between Paul's theology as represented in his letters, and the various beliefs in the stream of Second Temple Judaism. It is a huge question on where we should locate Jesus in this stream, but it's entirely possible Paul is much closer to Jesus than his critics tend to think.

Footnotes

¹ Daniel R. Langton, 'Paul in Jewish Thought', in The Jewish Annotated New Testament (2011).

² Ibid., p.586.

³ Ibid., p.587.

⁴ Mark Nanos, 'Paul and Judaism', in The Jewish Annotated New Testament (2011), p.551.

⁵ Ibid., p.553.

⁶ e.g. Richard Bauckham, God Crucified: Monotheism & Christology in the New Testament (1998); Larry Hurtado, One God, One Lord: Early Christian Devotion and Ancient Jewish Monotheism (1988, 1998).

⁷ Larry Hurtado, One God, One Lord: Early Christian Devotion and Ancient Jewish Monotheism (1998), p.99.

⁸ Ibid., p.125.

⁹ Pamela Eisenbaum, Paul Was Not A Christian: The Original Message of a Misunderstood Apostle (2009), p.173.

¹⁰ Ibid., p.177,178.

¹¹ Ibid., p.189.