For starters: The Matrix. When I was about thirteen, I saw the movie and asked my dad, "How do we know we aren't living in The Matrix?" and his reply was "Because we have The Truth." Later, I argued with my mother that there was a chance, no matter how small, that we were wrong about the truth. Her reply was that there was absolutely no chance that we were wrong. None whatsoever. When pushed, she finally agreed that there was only a 0.01% chance that we were wrong.
The other big one: In grade eleven, I read W.O. Mitchell's 'Who Has Seen the Wind.' It revealed to me the Problem of Evil and how the Christian God really is a logical impossibilty. Needless to say, that kinda clinched things.