I was pretty hardcore Christian for I guess about a year around age 14-15. I needed an elective in 10th grade and everything was full except home ec and “bible history” (should’ve been called “church,” in retrospect), so I took bible history, thinking it was “the right thing to do.” I grew up in the bible belt, so I had always sort of had this vague notion of God, and believed in him, but unbeknownst to me at the time, my parents were atheists (they’re both from California), so I was never hardcore about it. God was something I sort of took for granted and yet never thought about at the same time, if that makes sense. That year I took bible class though, I became pretty hardcore for awhile. I started going to church twice a week with anyone who would go with me, started carrying around a bible and quoting it to people; basically just being an insufferable little shit. By the end of that year I had read the entire bible (some books several times) and was sniffing bullshit. The teacher told us a bunch of stories that I gobbled up, but later found out were urban legends from a book that I found at a comic book convention of all places. Examples were the “missing day theory” and the story of geologists drilling so deep they could hear people screaming in hell, etc.
Pretty much the same sense of “doing the right thing” that lead me to take the class in the first place lead me to believe that this “God” character was quite the bully. It was Hell that did it for me, in the end. I couldn’t accept that a loving God would refuse to prove his existence, and still burn one’s soul for eternity if he or she didn’t take it on faith.
I can’t say any atheist argument convinced me, because I had never heard one, nor had I (to my knowledge) ever met an atheist until quite awhile after deciding I was one.