Didn't Paul invent Christianity?
No. Paul received the Gospel from those who were apostles before him, checked it with Peter and James, and preached the same message they did.
Paul says he passed on what he received: Christ died for our sins, was buried and rose. He visited Peter and James in Jerusalem, and they gave him the right hand of fellowship. The Gospels, written independently, carry the same message.
3For I delivered to you first of all that which I also received: that Christ died for our sins according to the Scriptures, 4that he was buried, that he was raised on the third day according to the Scriptures,
1 Corinthians 15:3-4 (WEB) · context
18Then after three years I went up to Jerusalem to visit Peter, and stayed with him fifteen days. 19But of the other apostles I saw no one except James, the Lord’s brother.
Galatians 1:18-19 (WEB) · context
9and when they perceived the grace that was given to me, James and Cephas and John, those who were reputed to be pillars, gave to Barnabas and me the right hand of fellowship, that we should go to the Gentiles, and they to the circumcision.
Galatians 2:9 (WEB) · context
15Regard the patience of our Lord as salvation; even as our beloved brother Paul also, according to the wisdom given to him, wrote to you, 16as also in all of his letters, speaking in them of these things. In those, there are some things that are hard to understand, which the ignorant and unsettled twist, as they also do to the other Scriptures, to their own destruction.
2 Peter 3:15-16 (WEB) · context