Well, here's another couple of ideas that don't even need anything supernatural.
Most of the NT we inherited is one way or another via Paul. Plus some later additions, but still in the context of his version. (Even gospels we now inherited as from Matthew or Peter seem to be actually later confabulations.) We also know that he had no problem with making up lies to proselitise, and he actually did it at least once. (2 Corinthians 12:16)
1. This guy called Saul is quite the zealot (and according to some the Zealot) kind of guy, and by his own accounts quite active in hunting down Christians and instigating that they be killed. Unfortunately he doesn't seem too effective, and their numbers slowly grow.
Then one day he gets a (Baldrick-class
) cunning plan: be even more active in spreading a massively distorted version of their teachings, to the extent of his even directly bad-mouthing the sect of the actual people who knew Jesus (now led by James and calling themselves the Ebionites). And I mean so different that, by what we know now, the Ebionites considered Saul/Paul an apostate. He even has to be rescued by the Roman authorities from them because they wanted to stone him... and that pretty much tells you how well his version fits with the version of those who knew Jesus in person.
Essentially, Saul nips Christianity in the bud by creating enough following for his own BS that the Ebionites would run into a massive conflict with later. He only doesn't foresee how successful his fiction will prove, but we can forgive him for that.
2. Almost the same setup: Saul is, if we believe that interpretation, a Zealot. Nasty fundamentalists and quite the anti-roman group. Saul is born abroad and a Roman citizen, so he probably is viewed with suspicion by a lot of other Zealots, but I'd assume for others he's the cute poster-child for the idea that even someone like that would see things their way. (Sorta how creationists nowadays love a former scientist who saw the light as their poster child.) In true fanboy fashion, he'd probably try to be even more rabid, to prove that he really does belong to their group. It would explain his earlier extreme zealotry.
Now Saul probably joined them earlier, in their militant but still moderate early days. Unfortunately, over time they grow more and more xenophobic. Eventually, rabid YHWH fanboy or not, he'd be the foreigner they all hate. It's sorta like being the black guy in the KKK.
His revelation would come after being rejected by them. And after all he's done for them. He feels betrayed.
In a common rejected-fanboy maneuver, Saul switches allegiances over night to opposing them. (There are no more rabid fanboys against Sony than those who were once rabid fanboys of Sony.) If heresy is what they hate, he'll show them heresy. And it's telling that he pretty much preaches ignoring the teachings of Judaism entirely, or even outright against them. In a complete reversal of the Zealot position, now he actually preaches favourably for the Romans and unfavourably towards the followers of Judaism.
He's not even as much in it for the religion of Jesus, as just against Judaism. If a perversion of Judaism is what it takes to get the "you don't have to follow the Torah any more" message across, so be it. He's just using the Jesus incident as a framework on which to build his anti-Judaism, and soon runs into conflict with the actual followers and disciples of Jesus too.
3. Saul actually has a "revelation" in a sunstroke or maybe after an Ergot-contaminated meal. (Ergot effects are identical to LSD, which is known to occasionally leave people convinced that they had mystical experiences and revelations.)
He doesn't have a clear idea at this point exactly what these Christian guys believe in, so he (subconsciously) mixes his own doubts and ideas with elements of mystery cults and doomsday cults that he was obviously familiar with. (And would be hard not to be.) E.g., the resurrection theme, as well as bread and wine being the body and blood of a god, are for example pretty much _the_ central element of the Osiris-Dionysus mystery cult.
He's probably genuinely convinced that he had a genuine revelation. To the extent that when he finally does learn what the Christians believe, he sticks to his own version.
