I was working in a call center at the time, and there were tvs around the room on mute with closed captions that we could look at in between being on a call. I actually didn't work in a department that took calls at the time, so I would watch entire shows like this sometimes.
Anyway, the trailer for this movie came on during a commercial break, and there was one scene (I forget the exact quote- I might have to check YouTube for it now, but you'll see it if you look it up) that was taking about the four horsemen, and mentioned that Christianity wasn't the first to use that metaphor, but that it has been used by a bunch of religions and people groups before.
So that threw me for a loop and made me start wondering. And once I figured out that one thing I always heard specifically about the belief was wrong, it made me wonder what else was wrong.
So once I stopped taking everything for granted as just being true because "the Bible says it is" and started looking at it honestly, it quickly turned into realizing that it was all bullshit. As soon as you stop assuming it's all completely true no matter what the evidence said, and actually take reality into account (for anyone that didn't grow up as a believer this sounds totally insane, but that's really how it be), a lot of things snap into focus.
I had a roommate I completely disagreed with at the time, but I started taking him a lot more seriously after that. Turns out he wasn't crazy, I was.
And that's the short version. I don't recall enough specifics to really make a long version, but you get the idea.