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. Verse 9:7 tells us the individuals traveling with Paul heard Jesus' voice but saw no one. Verse 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, Matthew 25:31-46, John 15:10, James 1:22-25, and James 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 Bultman
"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