In my case, atheism just seemed to come naturally. My family was not strongly religious, but we did go to services sometimes. I had some Sunday school religious instruction as a youngster. But even as a kid, I never really internalized it. It never made much impact on me. I think I went along with it just to get along. As a young teenager, when I started to think more philosophically, I realized I just didn't really believe what religion was saying. When I got to college, and studied some religion and philososphy, my non-belief was reinforced. And it's been that way ever since.