I remember pretty clearly that I was 17 years old. I had just started university. I had always found Catholic masses really boring so I skipped church the first weekend I was away from home. I met a girl who was Catholic and we were talking. My neighbours in residence overheard and so the subject came up later. They asked me, "Have you ever read the bible?"
"Well, yes, parts of it"
"The parts they read in church?"
"What about the whole book?"
So I grabbed the bible the next time I went home. Then I spent a lot of time reading it. All of it. I didn't immediately read for depth. I was flummoxed by the God doing all of the creating twice at the beginning of Genesis but glided passed that.
I was looking, skimming without realizing it, for the point where the impossible parts I'd always taken as metaphoric (the garden of Eden, Noah's Ark etc.) would be separated from the historical account of Jewish and then Christian history.
There was no such disconnect. The bible counts a direct genealogical linkage from Adam and Eve all the way to Jesus Christ. This floored me.
The Garden of Eden does not literally exist. The bible says that Adam and Eve were expelled and two angels were left guarding its gates so that the couple not return. If there is a gated garden, somewhere in the Middle East, guarded by two angels with flaming swords, someone ought to have noticed. It would be on google maps for one thing.
The fact that all of the "metaphorical" faery tales were joined directly, genetically, to actual history was a deep chink in the armour of my belief. The bible was trying to tell me that there was literally a Noah's Ark, literally a Garden of Eden, literally all of those other impossible stories. I had too much education to believe it, but there it was, a smooth temporal spectrum from Adam and Eve to Jesus Christ.
That made me read in more depth. Now that my belief in this was waning, I started seeing the cruelty, the capriciousness, the random punishment and reward for random acts of people. I began to judge. Everything, including the morality, seemed slapped together to support a framework of obedience to whomever was claiming to be on the side of God in each story. If God was all powerful, why was he creating so many people that apparently needed to be killed in the most awful ways? Why make people with homosexual urges and then call them abominations?
When I read the bible, I started to become an atheist.