I’m currently re-reading the Terry Pratchett Discworld series, and have made it as far as Hogfather, Pratchett’s decidedly warped perspective on old Father Christmas (or as we Americans would call him, Santa). For some reason this reminded me of a famous quote: “If you understand why you do not believe in everyone else’s gods, then you’ll understand why I do not believe in yours.” It’s pithy, but imperfect, because so many believers have really warped reasons for rejecting other gods. (Justin Martyr, for instance, once explained to Caesar that Jupiter and the other Roman deities were really demons pretending to be gods, in order to lead people to hell.)
With that in mind, I’d like to propose an updated version of the original quote: once you understand why you do not believe in Santa, you’ll understand why I do not believe in Jesus. I know, it’s flawed too (don’t try it on Jews or Muslims, for example). But I think it might be a bit more effective with Christians than the original.