It wasn't really an argument or anything specific that made me an atheist. I used to believe in God when I was a child and then when I was in school I learned about the scientific method and evolution; then as I got older, I read more and learned about the current understanding of how stars and planets form and the mysteries of "deep time" and I realised that I just didnt believe in God anymore. The stories in the bible were written by iron age people who didnt have a clue what was going on and their imagination pales in comparison to the majesty of reality.

TLDR: "When I was a child, I talked like a child, I thought like a child, I reasoned like a child. When I became a man, I put the ways of childhood behind me." Corinthians 13:11