If Paul was the originator of Christianity, where did all those supposed Christians he was persecuting before his conversion come from?
...
I think I know the likely answer but my evidence is weak *.
I think the people that founded Christianity were part of a religion that morphed into Christianity. I think the group was probably the God-fearer group that is mentioned in Acts and by Josephus a few times and for which there is some small amount of other evidence. This idea, if true, would explain a few things:
1. Who Paul was preaching to?
My view is that they were probably God-fearer groups and Paul was either pitching the idea of a crucified and risen God to them as the prime promoter of that idea or I think more likely the idea already was beginning to gain traction before Paul hops on the band wagon and he exploits an emerging market for information about this crucified and risen god.
2. How did the NT authors develop such a deep knowledge of the Septuagint (early Greek version of Old Testament) before there was even a Christian religion available to drive interest in the Septuagint?
An answer is that there was a group of priests in the proto-Christianity that had immersed themselves in Old Testament minutia. When the idea of the crucified and risen God began to take hold they used their deep knowledge of the Old Testament to work parts of the Old Testament into the stories they were writing about Jesus.
The idea of a proto-Christian group could be used to provide support for either the idea that an HJ existed (the HJ story was the seed that initiated the transition in the proto-Christian Group) or it could be used to support the idea that an HJ didn't exist (Christianity already existed in some form by the time of the hypothetical HJ so there was no need that an HJ exist for the formation of the Christian Religion). I don't think it does either. I think it provides support for the idea that it is unknowable whether an HJ existed or not. There just isn't enough information about this early group to know what happened as proto-Christianity morphed into Christianity.
As an aside I think this idea goes along with what eight bits said above about the use of the Gospels as a source of information about early Christianity. I think he was saying that the Gospels as a source of data about the nature of the HJ is of limited value (I would say of no value), but that the main value of the Gospels as historical documents is that they exist and that they can provide clues to the origins of Christianity.
ETA:
* It was Craig B in an earlier thread's challenge of my ideas about this that made me aware that the evidence behind my ideas on this was weak. I realize now it is weak, but I have now developed confirmation bias behind the idea so I am unlikely to change my mind unless there is some pretty good evidence that it is false.