I discuss religion with people and I often have to field the question of how I changed from devoutly religious to atheist. I figure I'll post it here so that I can refer people to it in the future.
I was raised in a strict Christian home. I was taught from birth about the Bible, taught to believe in God as The Creator, that Jesus was my savior, etc. I chose to be baptized at 12 years old (we didn't baptize at birth, it needed to be a choice) and was active in my church. I gave sermons in front of the congregation and actively preached to convert others in the community.
I attended college, much to the dislike of the church elders. At times it was really difficult to interact with college friends because my religious beliefs were not compatible with every situation. It often strained those friendships, but I stood by my religious beliefs.
The changes happened in class. When taking classes in astronomy, physics, history, etc., I would sometimes run into lessons that I felt contradicted my religious teachings. When this happened, I would spend time in heartfelt prayer and study to resolve the contradiction. I would come to one of three conclusions. Either:
If I came to the secular conclusion, I would replace just that one tiny bit in my belief system with the new idea. The thinking went something like, "Ok, maybe that one little thing is either a mistranslation, or was figurative instead of literal, or whatever, but the teachings overall are still solid regardless of this one little detail." Then I would continue on my way as a devout Christian.
This happened again and again, with little thing after little thing, over many years. At first it was about things learned in college classrooms, but then it moved into real world concepts like civil rights, politics, etc. Maybe 7 years of this go by and someone asks me what religion I am... I gave the same answer I had been giving my entire devout life and suddenly stopped and said, "Wait... no, that's not right." The other person was quite confused, because I really should know my own religion, right?
It occured to me that I had replaced, very slowly, so many of my individual beliefs that I could no longer identify myself as a Christian. I began to look at my relationship with Christianity. I looked at my sources of authority.
Why did I believe in the Bible?
Well, the most obvious answer was that my parents told me it was true. So, I began to trace the authority backwards. The Bible is a collection of writings from thousands of years ago. They often describe events that violate the natural laws of physics, they describe the origin of the universe and are often contradictory. If I was going to believe in the Bible, I would either need some proof of these fantastical claims or I would need some sort of believable authority. Proof wasn't going to be an option, so I looked for authority. My parents were my authority, but they had no real evidence or proof or first-hand accounting of the events. They believed because THEIR parents told them to believe as children. On and on, the cycle reached backwards. Each person in the chain only believed in the Bible because someone who held some kind place of respect had told them the Bible was true. Now, sure, some people had other reasons for believing a God or the supernatural, but their belief specifically in the Bible had to come from someone telling them that this particular book was the one true way. At this point, the Bible held no particular power for me. I still believed in God, but I had no reason to believe that the stories in the Bible were a true accounting of God. I started calling myself "spiritual" instead of Christian.
Later I started to realize that my original belief in God was also only due to childhood teaching, just like my belief in the Bible. Much like I asked myself "why do I believe in the Bible?" I started asking myself "Why do I believe in God?" I couldn't cite any reason for a belief in God other than "my parents told me God exists" and "The Bible says God exists." I eventually started referring to myself as agnostic.
The trend continued and I eventually came to a point where I referred to myself as "effectively atheist." I didn't know if there was a God or not, but I had seen no evidence that a God was taking part in my day-to-day life, so I was "effectively atheist." Perhaps God did exist, but wasn't the book-keeping, recording-all-of-your-sins-for-later-judgement type of God so it didn't matter if I followed a certain set of Holy Rules.
I tried opening myself to other types of spirituality with Buddhism and Daoism, but each time, I always came back to the same place and eventually started calling myself an atheist.
There was no big event that shook my faith or epiphany that sent me fleeing from religion or God. It was a slow, methodical journey to a much freer, happier life.