Short answer: Douglas Adams
Longer answer: I picked up a copy of The Hitchhikers Guide to the Galaxy in the library of a Catholic School I was visiting. (As Catholic schools go, I probably picked a
good least worse one). By ‘ “The lights had probably gone”. “So had the stairs” ‘, I was hooked. Which led to me reading The Salmon of Doubt, and then eventually Richard Dawkins. That’s probably how I became an atheist. Guess which misrepresentation (maybe unintentionally?) of The Selfish Gene we were taught in AS-level Philsophy of Religion? (But I didn’t read that until my 20s, or The Blind Watchmaker until I was at uni).
Sarcastic answer: and why did I become an atheist? Science can only answer how, not why. A-hah-hah-hah-hah. Hah.
Relevent answer: I eventually realised that I’d never really seriously considered if there was a god, or if Christianity was true, or if Jesus ever existed. I think I’d kind of accepted that it was true, and that I believed it, but not really thought about it. I remember ‘solving’ the answer, about why there wasn’t a god, but I don’t actually remember the solution. The existence of Jesus seemed like a hard thing to deny. Although I do remember thinking along the lines that if we only believe Jesus existed because the previous generation believed, we don’t actually know. Multiply that by ~ 4*20 = 80 generations, and there’s a reasonable grounds for uncertainty.
But I guess you don’t even need Jesus’ non-existence to deny his divinity, or his mother’s virgin birth (because then it would be her mother’s virgin birth, unless he got a Y-chromosome by magic). And I don’t need to refute Christianity, because Christians have no problem refuting $every_other_religion. Which doesn’t mean there isn’t a god, but it’s probably not anything like any of the religions describe theirs as, and it doesn’t exist.
Thinking about it, I’m only an athiest because I read a Douglas Adams book at a Catholic school I happened to be at, then DNA died at 49 and mentioned Dawkins in a talk published in essay-form (and an interview) in a book after his death, which I then read. That’s so improbable, there must be a god! Crap.