I was raised Jewish. My atheism arose in part out of the contradiction between the bible's creation story and the one we know to be true through science. As a young child, when I first heard about dinosaurs, I asked my mom why it didn't say anything about them in the bible, and she didn't have an answer for me. I quickly decided on my own that fossils were a test of faith. This was around the time I was 5. And I never really questioned anything for a long time.
I didn't really even think about it again until I went to a day camp at the Houston Museum of Natural Science, where everyone was perfectly confident in the existence of dinosaurs and several knew even small details about their anatomy. Feeling uncertain, I asked an adult I was close with what they thought about contradictions between religion and science. Thankfully, they told me that you just can't argue with scientific evidence. If they hadn't, who knows what path I would've followed. I was probably about 10 at the time, maybe a little older.
So from then on my goal was to reconcile what we know through science and the story in the bible. It was possible if you squinted your eyes a lot and didn't think about it too hard. The thing that ended my theism, though, was generally that I required a logical basis for belief (none of that "faith" stuff) and specifically my rejection of the first cause argument for God, which I had used for a while. It happened around the time that I was 14. I don't remember a specific moment when I stopped believing, but I remember feeling uncomfortable when people talked about religious belief for a while, and I remember some of the things I said to God shortly before I stopped believing entirely.
One comment: As I was raised Jewish, people have been throwing Pascal's gambit at me for a long time, and it's never stopped disgusting me. You may think I'm going to Hell because I don't believe your dogma, but the ancient Greeks probably thought you were going to the really bad part of Hades because you never sacrifice cattle. Who cares? I certainly don't. So stop using that argument unless you want to start sacrificing cattle "just in case."
Oh, and one more clarification about what I believe and why: I don't believe that the universe just came into being out of nowhere, either. Nice strawman, though. God isn't the only explanation; certainly not a very good one.