If Paul didn't create Christianity, then there could have been a Jesus who created Christianity. Paul is much more likely the creator, though, because we have conclusive evidence that Paul existed but no such evidence for Jesus.
I don't see conclusive evidence that Paul existed. I see conclusive evidence that the bottle of grapefruit juice in my hand exists, Paul not so much.
I'm confident he did exist as a individual. It's far and away the most plausible explanation for what we do have solid evidence about. But it's possible that Paul was an invention.
That's correct, but there are some passages in Paul's epistles where he seems to suggest that Jesus did spend some time as a real man on earth. The aforementioned 1 Corinthians 15, for example.
Again, this is all a matter of plausibility. The most plausible explanation, to me, is that an unusually charismatic individual named Jesus inspired something that grew. Paul discovered it for himself, and flipped from persecutor to proselytizer. With the fervor of a convert Paul started spreading the Good News. He spread it far enough to prevent it from destruction by the Romans. He probably elaborated on it and helped it start becoming the epic myth it eventually became.
If all that evidence for Jesus was destroyed by the Romans, then how do you know it ever existed?
I don't claim to know much of any of this. I'm describing what I find most plausible and why. I know that the Romans bulldozed Jesus's world within a few decades of His life. Whatever authentic information concerning His life existed probably only as an oral tradition.
Also important is that Jesus was a convicted criminal, according to the Romans. Convicted of treason against Rome. Neither He nor anyone who claimed to revere Him could ever talk openly about the historical Jesus. Or they might wind up on a cross too. That would make it both easy and needful to start creating a legend around Him.
Who started the "Jesus movement" and why rule out Paul as its inventor?
I'm not ruling out much of anything.
I also doubt that the Jesus Movement and Christianity had much in common. A generation or two of inter-cultural "whisper telephone" turned the Jesus Movement into Christianity. I don't think Jesus would recognize, much less own, the Greco-Roman epic myth Christianity revolved around.
Tom