Next Saturday marks the beginning of the end of days. Or at least that is what the Christian radio broadcaster Harold Camping and his many followers think. By Mr. Camping's calculations, May 21 is the day when Jesus will return to judge all of humankind and to gather up the faithful.

But what will happen the following day, when this prophecy falls short? Doomsayers almost never admit that they were wrong; they usually become even more adamant about the truth of their beliefs,...