As part of an independent study in existentialism, Fear and Trembling by Soren Kierkegaard.
By his reckoning, I never lost my faith in God, it's just that I never had it in the first place. I'm comfortable with that definition, though. I don't think I'll ever be a knight of resignation or a knight of faith, and Abraham from the Bible is not my ideal. That's the cool thing about existentialism: the author advocates a particular way of life that they claim will prevent despair, but if you disagree, you don't need to come up with a logical argument as to why their argument was wrong because existentialism isn't rooted in logic anyway.
As an aside, Kierkegaard seems to have had a whole host of psychological problems, but at least he was really smart.