I think even if you killed Jesus early enough, given the climate of the times and the fact that every other person claimed to be a messiah that a new religion would be created regardless of who it circled around. I'm guessing that one would have been just as caustic to mankind as Christianity.
Even if the open windows of science at first make us shiver after the cozy indoor warmth of traditional humanizing myths, in the end the fresh air brings vigor, and the great spaces have a splendor of their own - Bertrand Russell