I was lucky, I wasn't brought up in a religious household, so essentially I've been an Atheist all of my life.
I never thought much about it until around around 2000, when the internet was on the rise, especially Youtube.
I watched a lot of debates about religion, Science, and creationism.
So I figured,I woul;dn't get involved in any debates until I read the bible all the way through.
The whole time I was thinking, are you serious, people actually believe this crap.
It was my last and final reading of the Bible that convinced me that the entire narrative was concocted. I was brought up in a religious (school / church / social) community, where we memorized Bible verses and entire chapters, and could rattle off the names of the 66 books of the (Protestant) Bible by the 2nd grade. And yet, what a relief it was to finally arrive at the logical conclusion that the entire thing is a crock of shit—works of fiction nowhere near as good as Homer.
I get told that quite often, and people are always surprised. IMO, they should be embarrassed - you believe this is the WORD OF GOD and yet my blasphemous godless self knows it better than you do! I agree with several posters - it is usually what convinces people what a crock it all is. Maybe that's why believers don't know it as well - they're afraid.