I was raised Catholic but I'm now agnostic.
I started losing faith in the religion in my mid-teens, when I actually started to think deeply about the world around me, and about the religion itself. I never felt that spiritual spark that everyone else seemed to have. Never got over the inconsistencies of inadequacies and contradictions of religious belief. I think learning about history, the history of society and the history of religions, is one of the main reasons I left the flock.
I don't try to evangelize my point of view, because it took me years of living and learning, and a very personal lack of affinity for religion, to get where I am. I have no problem discussing my point of view, however.