A lot of people, Christian or not, have great respect for Jesus. He may be seen as a great ethical and religious teacher. He may even be considered a prophet of some kind.
Some of those who have respect for Jesus, have much less respect for Christianity. They like Jesus but dislike some of the theological assertions of organized Christianity.
Where did the church go wrong? For some the answer is clear: Paul.
I have read criticisms by Muslims who have suggested that Christianity veered off from the teachings of Jesus because of Paul’s innovations. I have read similar things by some Jewish writers who wish to reclaim Jesus for Judaism.
It is a valid historical questions to ask whether Christianity as it is now finds its origin in Jesus or Paul.
One of the challenges in answering this question is the dating of our sources. It is likely that Paul’s letters predate our written Gospels. How can we tell if Paul changed anything if his writings are the earliest that we have?
I would say that the burden of proof is on those who insist that Paul changed the teachings of Jesus and introduced new ideas. Having said that, I believe that the weight of the evidence is on Paul being consistent, if not with Jesus, than at least with the earliest Christian teachings.
For example, some of the distinctives of Christianity include the resurrection of Jesus and divinity of Jesus. These are found throughout Paul. But did Paul come up with these ideas?
Paul seems to cite in 1 Corinthians 15 an earlier creed about Jesus’ resurrection. There is evidence that the clearest statements about the divinity of Jesus in Colossians and Philippians are earlier creeds that Paul adopts.
Paul also addresses the question of whether he had a different gospel in what is likely his earliest letter:
But from those who were influential (whatever they were makes no difference to me; God shows no favoritism between people)—those influential leaders added nothing to my message. On the contrary, when they saw that I was entrusted with the gospel to the uncircumcised just as Peter was entrusted with the gospel to the circumcised (for he who empowered Peter for his apostleship to the circumcised also empowered me for my apostleship to the Gentiles) and when James, Cephas, and John, who had a reputation as pillars, recognized the grace that had been given to me, they gave to Barnabas and me the right hand of fellowship, agreeing that we would go to the Gentiles and they to the circumcised. They requested only that we remember the poor, the very thing I also was eager to do. (Galatians 2:6-10)
Of course, this could be Paul trying to cover up his involvement in changing Christianity. But we would need positive evidence for that before dismissing what he says in Galatians.
What about the conflict that Paul seemed to have with some Jewish Christians?
It is true that Paul did have some issues with some Christians from a Jewish background. But that conflict was not about what we would consider the key doctrines of Christianity. The question was about the process by which Gentiles would become followers of Jesus. Were the Gentiles required to convert to Judaism first or were they able to directly follow Jesus? It is interesting that Luke reports that it was not through Paul, but rather Peter that the Gentiles first were able to directly follow Jesus (Acts 10).
Was Paul influential in developing Christian doctrine? Absolutely. Much of what Jesus taught was aimed specifically at a Jewish audience. There were many topics that Jesus gave no clear teaching on how things worked in the kingdom of God. Paul took what Jesus taught and applied to a Gentile context.
However, I would suggest that evidence is lacking that Paul created his own form of Christianity distinct from the teachings of Jesus.