Two things started the slow 10ish year journey to atheism for me. I can't remember which happened first.
Some Mormon lads doing their mandatory missionary work knocked on our door when I was home alone. I decided, screw it, kill them with kindness. Maybe I'll convert them! After I got them some ice water, they started the spiel. It was so stupid, how could anyone believe this? Then I thought, wait, how is what I believe any more believable? That was an unsettling thought that I could never really shake.
I also challenged myself to read the entire Bible (NIV) front to back (which I did, thankyouverymuch). I already had a lot of apologetics for the pentateuch warfare, slavery, etc. but in Psalms there's a verse that basically goes, "blessed is he who dashes the babies on the rocks." And like. What the fuck is that. In what possible circumstances is killing babies okay, let alone with God's explicit endorsement? That also stuck in my head ever since.
There was a lot else in between, but years later I stumbled into a copy of The God Delusion. "Know thine enemy, right?" So I read it on lunch breaks at work. While I now know the book has a reputation for kinda bad philosophy, by the end it had tidily dismantled the last vestiges of the purely "rational" arguments to believe in God I still had. So I sat there, an atheist for the first time in my life.