It's a defense mechanism. Rather than admit that a Christian finally realized they were deluding themselves, they have to claim that the Christian wasn't really a true Christian. What they feel through god is the real deal, so anyone who would give that up just wasn't doing it quite right.
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
