It is not black and white for everyone. And it is not a constant through life.
I used to believe, and quite strongly in my opinion. I not only read the bible, but also a lot of books by theologians, and I was going to church every Sunday. Now, I am 100% an atheist, probably as much as Dawkins. (By the way, his books are great, including The God Delusion. )
In my case, nothing tragic happened, it wasn't so much a single instance when I stopped believing, but a gradual realization of how the world works, that we create myths so that society can function, how Physics and Evolution shaped us and so on. Science explains a lot of what in the past was a mystery. And of course, I realized that the priests take advantage of our need for meaning. And in the past they used to kill everyone that did not believe, so we are "breed" to have faith in god.
I know that a lot of believers are great people. Myself, I don't believe, but I don't dislike people who believe in god, as I don't dislike people who believe in astrology.