It's not rare to encounter people who claim that Paul "invented" Christianity. The basic idea is that Jesus taught a pure and ethical form of Judaism that focused on God and gracious living, while Paul developed a religion that worshiped Jesus rather than God. Though this idea literally makes no sense historically, it's gotten a lot of run. Even the occasional serious academic book "blames" Paul for perverting Jesus' message in inventing Christianity.
One easily appreciates the appeal of this position. In the first three Gospels -- Matthew, Mark and Luke -- Jesus speaks continually about the kingdom of God. He does not ordinarily speak about himself. In the fourth Gospel, however, Jesus talks about himself all the time. Even ancient Christians recognized this phenomenon. Writing around the year 200, Clement of Alexandria described John as "a spiritual Gospel" on the grounds that it relayed not the literal history of Jesus' career but its spiritual and theological significance. How did followers of Jesus move from a religion focused upon Israel's God and God's kingdom to a religion devoted to the person of Jesus? For many, the Apostle Paul fills that gap.
However, every bit of evidence we possess demonstrates that Paul did not, in fact, invent Christianity. Let's begin with how Paul came to follow Jesus in the first place. The book of Acts claims that Paul, having already persecuted some believers in Jesus, has a visionary encounter with the risen Christ. Paul himself describes that encounter as an "apocalypse," or a revelation. In any event, Acts agrees with Paul that the new apostle turned for support to a community of believers that already resided in Damascus.
If Paul invented Christianity, how did that community in Damascus come to exist? Paul's "conversion," as some call it, occurred within just two or three years of Jesus' death -- and already communities of Jesus followers were spreading beyond Judea and Galilee into Samaria, Syria and other parts of the ancient Mediterranean world.
Moreover, a look at Paul's missionary career debunks the notion that Paul invented Christianity. Having joined the believing community at Damascus, Paul later goes on to Syrian Antioch. The believing community there -- Acts refers to them being called "Christians" -- supports Paul and his partner Barnabas in their missionary activities (Acts 11:19-26). Obviously, the church would not have supported Paul if his teachings represented a radical departure from what they already knew. ...
So we have a pattern. From Damascus in southern Syria, to Antioch in northern Syria, to Ephesus in Asia (today, Turkey), to Rome and hopefully on to Spain, Paul extends his missionary work to embrace the entire northern Mediterranean rim. As he does so, he relies upon churches located in major cosmopolitan cities to support his mission. All of these churches existed prior to and independent of Paul's mission, yet they support him. This could not be the case were Paul inventing a dramatically new interpretation of Jesus.
Read it all.
I note also C. S. Lewis' thoughts from 1947 on that "most astonishing misconception [that] has long dominated the modern mind on the subject of St. Paul" to the effect that "Jesus preached a kindly and simple religion (found in the gospels) and that St. Paul afterwards corrupted it into a cruel and complicated religion (found in the epistles)." Lewis addresses that misconception in his introduction to J. B. Phillip's translation of the New Testament epistles entitled Letters to Young Churches. You can read it in my posting entitled "Impeaching and Banishing St. Paul."