C.S. Lewis, I think, failed to adequately account for the likelihood of stories propagating by exaggeration. Jesus need not have been a liar, a lunatic, or the lord, he could have been an honest, sane person to whom people ascribed claims of being divine after the fact (although as a religious leader setting up a splinter movement that strongly deviated from existing doctrine, I think the odds favor the historical Jesus having been at least somewhat crazy.)

I would say that the body of evidence posed by other religions suggests that, in the absence of a true religion, people will still make up stories of a religious nature (also, the degree of theological uniformity that exists among most existing strains of Christianity comes, not from the fact that early sects were at all unified, but that modern sects are almost all descended from the strain that killed the other ones off.) But my position is probably shaped to a significant extent by personal experiences with other people elaborating on outlandish lies that I came up with when I was young, with practically nothing to gain from it.