This is ganked verbatim from an article at infidels.org.
http://www.secweb.org/asset.asp?AssetID=313
"The followers of a probably basically decent street preacher/Essene/rabble-rouser who's just managed to get himself done-in by the military empire currently knocking heads (apparently at the behest of the local clergy, who were finding he was hurting their box office) are consoling themselves over some warm red wine. It becomes fashionable among them to comment that really, old Jesu is still with us, in spirit; he's not dead, really.
"A few years later, followers of the surviving movement are not so much repeating that his presence is one "in spirit" anymore. "He's not dead" then shifts to "he didn't die--it didn't quite take, not that those Romans didn't give it a try."
"They're thinking it's a bit more concrete than before. Inward, emotional, and fondly remembered perceptions, of the absent preacher's presence become understood as being a bit more physical. Again, it's probably still a mix of alleles, meme-wise; there are still people around saying, no, he wasn't actually here here, but we felt like he was. But as their story is a bit less interesting in another environment where the fantastical, for whatever reason, travels well, it doesn't get passed along as frequently as the other story.
"There are reasons other than just that it's fantastical which might promote the spread of the more-interesting versions of the story as well. Comparative religion types have previously noted the correspondences between many of the major religions, borrowed elements, and so on. The Christian story does bear some interesting resemblances, or at least apparently so, to the Isis/Osiris myths. Gods that sacrifice themselves and return from the dead would seem to have been something of a tradition. (As this is idle musing, not so much a scholarly article, I'm not going to go looking for footnotes right now). Again, you can imagine reasons, mechanisms that would have fused the tradition with the younger, growing story--adherents of both systems unconcerned to keep them particularly separate. (we still often hear the "it's all the same god" ecumenism stuff attempting to bridge traditions even today.) And leaning on its now archetypical, familiar quality, its tendency to spread grows yet again."
- AJ Milne
Basically, I think Christianity is still around because it's a strong, strong meme. It's much more attractive than some other religions that require you to do a set of things to win entry to Heaven; all you do here is believe and follow a by all accounts fairly awesome dude. Of course, those accounts were written decades after the fact, once memories had time to soften... and historians generally believe that Matthew and Luke based their accounts on Mark and another (lost) source we call Q. Whatever the truth about the man originally was, Christianity is likely no more than an elaborate cult of personality.