Look up the clergy project. Preachers who've spent years teaching and preaching the bible turned atheist. From people like Dan barker , Jerry Dewitt , and so on.How does one become an atheist? No it's not a trick question.
One simply becomes an atheist when they realize they can't hold on to the beliefs they were taught as a child (indoctrination) anymore and comes to terms with it. You are COC most likely because your parents were COC just like my Pentecostal friend is so because his parents were Pentecostal and I was baptist because my parents were. Had your parents not indoctrinated you as a child and let you decide for yourself when your mind had developed to a stage where you were capable of thinking rationally for yourself, you would most likely not believe either.