Well, as Stapel gave a serious answer, so will I, just to avoid a certain impression that is very common, that is that atheists "left God" due to a traumatic experience of some sort.
I was born and raised catholic, have studied religion in my mother's church, and have made my first communion and everything.
When my mother sent me to study in that church, she had no idea of the mistake she was making, because through living next to people so closely related to God and his ways, I managed to realize that they didn't know any better than every other mortal.
So, the aura of divinity and dignity that priests once had were, for me, shattered. They were just like other man, and as I realized that, I begun to realize that if the man that speaks for God, today, are not special, so maybe the man that spoke for him 2000 years ago also weren't.
Than came the most important event; in an afternoon, a num said the most idiotic thing I ever heard in my life; that we were to watch our thoughts very closely, because even thinking of sinning was sinning already.
I instantly realized the silliness and the danger of such idea. Police for thoughts. I couldn't believe that there were people in the world trying to install breaks in my mind. I always treasured free thinking and rationality too much to accept that sort of claim. And, from that day on, I was lost to religion forever.
Free from strains, and now without any previous set of information that I had to validate, walking the path from non-practicing catholic, to faithful without religion, to pantheist, to agnostic, and finally to atheist, well, it was a matter of time. Because when we look with impartial eyes, it's hard to validate any claim religion, or faith, makes.
Well, that's my happy little story. I guess when my mother sent me to church in order to give me spiritual guidance, she would never really imagine exactly how sucessful she would be...
Regards

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