Most nonbelievers seem to have grown up religious, later deciding for themselves that the theistic mindset is wrong, either through life experience or just general knowledge searching.
I never went through that. The first books I learned to read were from Greek mythology. I thought of them as superheroes rather than deities. I equated them with Santa, that they didn't exist. As I got older, churches only reinforced the idea. The attitudes and certainty of believers furthered my convictions. As I researched through high school, I only added reasoning behind the truth I already knew.
It's only struck me recently how unique, or uncommon, my experience was. This passage from John Stuart Mill's Autobiography resonated with me for that reason.
"I am thus one of the very few examples, in this country, of one who has, not thrown off religious belief, but never had it.... This point in my early education had however
incidentally one bad consequence deserving notice. In giving me an opinion contrary to that of the world, my father thought it necessary to give it as one which could not
prudently be avowed to the world. This lesson of keeping my thoughts to myself, at that early age, was attended with some moral disadvantages."