Probably the biggest defect of the theory that the biblical stories about Jesus were all fabrications; is the fact that most of the early Christian leaders (including Paul, Peter, and the likely authors of the Gospels)went to horrendous martyred deaths for the the sake of something they supposedly made up. To put it mildly, it's stretching credulity that somebody would willingly submit themselves to being boiled alive in oil or crucified upside down for a professed belief which they knew for a fact was false.