1. Learning about evolution and eventually believing it. At this point, I still believed in God. I saw no contradictions.
2. Learning about other religions and how similar they all are. Started thinking about why they exist, what purpose they serve people psychologically.
3. Critically examining the Bible. Learning that "I don't know" does NOT equal "God did it" or "It's magic." This was a huge step. When I applied this rule to everything, I found that the supernatural claims of religion are unfounded.
4. More research on science, philosophy, and history. Astronomy probably had the biggest influence. Lots of good refutations for design. And philosophy, like the problem with evil, what actually defines God, etc. Also, the way religion and God relate to fear of death.
5. Right now, posting about it...