Before I begin, allow me to make something clear.
I am an atheist.
If this offends you, I am sorry. I do not intend to insult or convert anyone, or else I probably would have spelled 'atheist' with a capital A. I merely desire to express my viewpoint as a member of a minority group.
The fact that I do not believe in any god, goddess or other deity does not mean that I deny the possibility that there is one (or more). My disbelief in God is similar to my disbelief in ghosts. I do not think it likely that the spirits of the dead return to haunt the living, but I will still run for my fucking life after turning off the lights in the basement. Utter conviction in anything just does not strike me as healthy.
No specific event caused me to lose my faith. I do not even remember when it happened--sometime between kindergarten and third grade, I think. It was not a particularly major transition: I do not recall ever attending a church service up until I was eight, and that one was for my grandfather's funeral. If there is a Bible in our house, I have yet to happen upon it.
I am not bothered by religion; in fact, I have a (some might say ironic) fascination with it. I am particularly interested in ancient religions, such as those of the Mesopotamian civilizations, the Egyptians, and the Greeks. After overdosing on Norse mythology, I decided to revert the days of the week back to their origins (Odin's-day [Wednesday], Thor's-day [Thursday], etc.). I once researched the Celtic gods with the sole intent of writing an epic in the style of the Odyssey. As one can imagine, I didn't get very far with that--two damned pages, in fact. And I lost my notes.
Recently, I came into possession of a copy of Bulfinch's Mythology (collected by Thomas Bulfinch, 1796-1867). While skimming through it, I noted an interesting bias on the part of the author, which I can assume was held by many educated Christians of his time. While he mildly criticizes the Greek and Norse people for the crime of polytheism, he describes the Celtic people as "immersed in the darkness of heathenism" (Bulfinch, 363).
I'm not sure that fact is relevant, but I thought it was intriguing.