I find it easiest to believe that he did exist, they thought he was dead but he wasn't, so they thought he had been resurrected. It would make sense that Jesus himself would have thought he had been resurrected if he had an NDE. You can imagine how that would have spread, that he had been resurrected, and been the start of Christianity. And Pauls experience of the light and loud voice that asked him, Why are you persecuting me. Certain brain conditions can make a person see a bright light (that isn't there) and hallucinate. That could have been what happened to Paul. He was actively persecuting Christians, so his mind would have been on that subject, which could explain why he hallucinated about that subject. Just my thoughts on the beginnings of Christianity.