What did it for me was a thoughtful re-reading of the Bible in my 40's. I became somewhat nauseated, and quite incensed, so that when I finally accepted the fact that the whole thing is made up, it was as though a huge weight had been lifted.

Simply asking questions, and thinking really hard about it late at night. I'd spend my days hanging out with friends, and feeling cool for being Christian. Until one day I started asking legit questions and when there were no answers from Christianity, I just decided to find something that did answer my questions.

The inability of religious faith (belief in the unsubstantiated) to accurately predict or explain experienced reality.

That was the precipitating factor. After that, I will go along with @Donotbelieve: facts. Some things, once known, can't be un-known.

1. No evidence.
2. Claims made in the bible are simply not believable.