Commenter Viggo the Carpathian poses this question:
When did you know you were done with religion?
“What, if anything, was your defining moment of unbelief?”
I was raised Christian and my moment came at the point of trying to join a church. They asked me if I was transferring from another church and I said, “No.” They said, “It’s a profession of faith then” and made a mark on their little form.
I froze on the words “profession of faith.” I never went back. I have not been to a church since.
(For what it’s worth, that may not necessarily be the same moment you became an atheist.)