The Rise of Christianity by WHC Frend is a good place to start really getting a handle of the history of Christianity, some of the decisions that were made, who the early players were, and how much it has changed since it's beginnings.
You can usually find it on Amazon but I don't think it's available digitally. If you belong to a church with a library, you might find a copy there. That's were I ran across it, before I deconverted and walked away from the faith.
To be fair though, Paul never knew Jesus so his knowledge of Jesus' teachings were all second hand knowledge. Nor did he write any of the Gospels or the book of Acts (that one was written by St Luke the Evangelist).
His entire ministry is based on a vision he claims to have had on the road to Damascus to extradite some Jews for trial. He never sat at the feet of Jesus like the disciples did and learned or heard the teachings of Jesus.
So it's a little incongruous to say he changed Jesus' words. What he did was Romanize a new faith.
It's not for nothing that Paul was often at odds with Peter (a disciple, the rock upon which said he would build his church).
Most of the new testament was written by Paul but the books attributes to him are not sacred texts. They're letters; to study the new testament is to study the post. Letters written to nearly churches he had established or visited. As such, most of the contents are less spiritual truths meant to be set in stone and more practical matters of how Christians should behave and how to resolve conflict in those early organizations, or suggestions. That's one of the reasons that, taken as a whole, the bible is so contradictory. Much of what was later canonized as sacred texts with the authority of being God inspired were letters encouraging new believers and giving church specific advice. Not all of it was meant to apply to all christians everywhere.
For example, Paul tells oke church it to allow women to preach, yet he himself preaches, converts, and performs miracles alongside several women, including Thecla.
