Finally, why didn't any of the other unorthodox accounts start similar communities? Why are the communities so similar in their beliefs about Jesus, if it is quite likely to have been made up, as you suggest?
Paul's letters explain this. The communities were so similar because they were all so strongly influenced by Paul.
I think LWers may be intrigued...
Tim McGrew, author of this excellent annotated bibliography on Bayesian reasoning, recently co-authored with his wife Lydia a Bayesian defense of the resurrection of Jesus. I interviewed Lydia for my podcast, here. Atheist Richard Carrier has leveled some objections to their article, but his objections are weak.
Have at it.