You're assuming that there were leaders. Not really, the earlier christianity was the combination of several loose teachings taken from different sources, including jewish and pagan. They were then combined by Paul, who preached a celestial being, not a human, as their ultimate source. His writings were later harmonized with the tale of a human Jesus, who appeared on a somewhat remote past and in a remote country that had been destroyed by the Romans (Judea). So, no wonder that nobody disagreed about this tale, nobody had really seen it happening other than by "revelation".
You're also claiming that the proto-Christians weren't working within a messianic Jewish framework (which the academic view of early Christianity fits nicely), but a completely different framework that we neither have evidence of before and that completely disappears afterwards.
Speaking of completely disappearing afterwards - Paul's letters are from ~50 AD, Mark is ~70 AD. So you have still have to posit a massive and seemingly unified (even though the early Christians were hardly unified) agreement to adopt the human narrative, since soon after we get a lot of information about heresies, and lots of criticisms about Christianity, but no one is aware that it (supposedly) was as a completely separate religion within living memory.
It's just a very strange theory, one that posits a completely different religion suddenly appearing and disappearing, and seems to be based solely on ignoring most of the content of the Pauline letters, twist the words of some of the others, and just making up the rest whole clothe. It's about as historical as Dan Brown's version of Christianity, and has as much scholarly support.
It's not built entirely on the letters of one person. Anyone who's studied early Christianity knows that there's a large variety of early sources, including Christian proto-orthodoxy, other Christian sects (including things that get labelled as heresies and gnostic today, though those terms are misleading), and early critics of Christianity.
This all gives us a decent understanding of how the religion developed during the first couple of centuries. Though we see a great deal of variety in early Christian thought, we don't see any of the evidence of this proposed precursor religion. The idea seems to entirely exist because people want it to, which is why it's been rejected by historians as a fringe theory.
I brought up the letters of Paul because the claim was that Paul personally created Christianity on his own as an entirely separate religion (one where Christ is a celestial and not earthly creature), and that it was completely changed (by...someone?) within a few years of his death. I'd think Paul's writings directly contradicting this theory would be rather important, not "dogmatic bias."
> myths are created all the time and always has been.
I can't think of any religion where historians believe the found preacher was invented within a few decades of it's creation. And the existence of myths in the past isn't carte blanche to ignore evidence and then make up one's own theory that's completely devoid of evidence.
No, you don't need agreement for any of this to happen. In fact quite the opposite, we know today that original christians (at the time of Paul) were at odds in practically anything, including the idea that Jesus came in flesh (you will find this being condemned in the new testament itself).
However, from several branches of this fringe cult one won out over time, the one started by Paul (who never knew a real Jesus, as he confess by the way). This one branch created a single story (the Gospel of Mark) that was copied and embellished by several others. This is the one version of Christianity that became famous and powerful.
So we don't have them all agreeing on the basic facts, as you mention. We have a story that was created and embellished, and them every other dissenting voice was suppressed.
What are you saying is being condemned in the NT, exactly?
> Paul (who never knew a real Jesus, as he confess by the way).
Where does Paul confess that he never knew a real Jesus?
In Galatians 1:12 to 1:16, Paul affirms that he didn't lean about Jesus from flesh and blood, but from God's revelation. In no moment Paul says he ever saw a real Jesus, only his "divine revelation", which is a big confession from someone who lived in Jerusalem during the supposed ministry of Jesus. Moreover, given his age he should have been himself among those who killed Jesus, but he never mentions this little fact.
Anyway, we know a lot about the beliefs of various early Christian groups, and even have the writings of a number of them. But none of them match the beliefs you're positing.
It's really bizarre when you think about it. There are figures that historians think might have been composite figures developed over centuries, but nothing like disparate groups suddenly creating a fake founding figure from a generation back (the Pauline letters are written a couple of decades after Jesus is supposed to have died).
And the theory doesn't solve any problems. Instead of one preacher we have some evidence for, we now have multiple preachers we have no evidence for, multiple proto-Christian movements we have no evidence for, councils to unify the movements by falsify their origin we have no evidence for, etc.