I come from a family that I guess you would call Catholic. My father's parents are Baptist, but he's rather apathetic to religion, much more so now since my parents know that my brother and I are both atheists. So the religious aspect of our life was dominated by my mother, who had gone to a Catholic high school. Although she wasn't really dedicated to Catholicism, much more to just the general picture of Christianity, seeing as part of my early childhood we went to a Methodist church rather than the Catholic one we later went to, and then even later we went to one of those non-denominational, "worst concert you've ever been to" churches.
My early church life was probably what's typical for most Christians. You go to Sunday School, learn about Noah's Ark and such, don't really believe it, but don't really think about the fact that you don't believe it. I really just didn't think about religion at all for the first 10 years of my life. I used to read the Sunday comics in church, which is a little odd because to this very day, I've never laughed at a comic in the comics section.
My religious life changed when I was about 11 years old. I remember my mother was in my 9-year old brother's room trying to comfort him because he was crying about something. When my mom came into my room I asked what was wrong, and she said that he was worried about dying. Perhaps most 11-year olds wouldn't be affected, but in my youth I was a huge worrier. I worried about everything, especially about my health. I worried about cancer, leukemia, brain tumors, internal bleeding, brain damage, and even AIDs. From this and other behavior that I had as a youth I think I probably was obsessive-compulsive, although luckily I don't have the symptoms anymore at all (*saves post as draft 8 times for good luck*). Just kidding, but that would have been me before.
Anyway, one of the ways that I stopped from worrying was to research what was bothering me and convincing myself that it was extremely unlikely that I had whatever problem I thought I might have. When I started worrying about death (eventual death, not immediate death due to some cause) my mother tried to comfort me by saying that I would be going to heaven when I died, in essence living forever. This is where my OCD (self-diagnosed, in retrospect) really kicked in for me. Now, in order to stop worrying about death, I felt that I should really explore this idea of heaven and be able to convince myself that I was indeed going, and that this place did actually exist. I wasn't going to simply take it "on faith" because like I said, I was a huge worrier. It had to be overwhelmingly clear that I didn't have a problem. So at this point I decided that I would more actively think about my religion, with the hope that I would soon convince myself of the near certainty of heaven, Jesus, God, etc. It didn't quite work out that way.
I started to pay attention in church, which is probably the worst thing you can do if you want to keep your faith. The preacher would sometimes say something that was either racist, something against science, or some other absolutely ignorant statement. This bothered me, and I attempted to discuss it with my mom on the way home from church. They were legit criticisms, so she couldn't really defend them. She'd usually just say the typical "god works in mysterious ways" or "you just have to have faith" sort of garbage. The more and more I saw this excuse, the more I translated it to be "yes, you're correct, our pastor is psychotic, and that part of the bible doesn't make sense at all". So after only a few months of this increased attention in church, I began to seriously doubt the competence of the preacher, and I was probably also a little angered by some of his statements. This was convenient because it put me, at least in my head, opposed to him. And when you're opposed to someone, you usually put in a lot more critical thinking into trying to debunk what they say. I should say that by this time I probably had moved on from worrying about death and was probably worried about some skin-eating disease or something instead, but now I was focused more on religion.
So being opposed to the preacher, I would listen to him talk and actually think critically about it. And so you can probably imagine how easily I began to doubt the most blatantly stupid of the stories in the bible, like the one of Jesus cursing at a fig tree for example. After enough of these examples, I just decided that organized religion was completely wrong. I didn't really believe anything that the Bible contained, although I held on to the vague idea of Jesus dying for our sins and the existence and nature of God. I probably held this general state of belief for a few years, until the age of about 14. I don't remember what prompted it, but I began to think about these issues again. I thought about the necessity of Jesus coming down to save people. My thinking was as follows
The night I thought this, I said to myself "omg, I'm an atheist". Instantly, the connotation of "devil-worshipper" and feelings of having done something wrong came to mind, because that's what I had learned to associate with the word atheist. I realized that these feelings and associations were completely unjustified, and this made me doubt religion even more for having made me think otherwise.
So that's pretty much the entire story. While my atheist position has surely become more sophisticated with time, reading of books, and reading of all the great blogs in the atheist online community, my actual deconversion was complete.