Like most Americans, I grew up with Christian parents & went to Christian Sunday school. The only ones I remember were side events of the various on-base churches we went to (my father was Air Force). Children attended the first 10-20 minutes of the service then were taken off to another room for Sunday school. From what I remember, it wasn't much more than the telling of Bible stories, usually with the assistance of puppets.
I understood that it was a way to keep the children occupied while the adults did whatever they were doing. What I didn't understand was that it was supposed to be something more than that. I never had a specific moment where I figured it out, but somewhere by the time I was 10, I had realized they thought they were telling me true stories.
I'm sure some may say it was stupid of me to go so long without realizing the stories were supposed to be true, but I don't see that I ever had any reason to see those stories as any different than the fiction stories I was told in other settings. I don't remember ever being specifically taught the difference between fact & fiction before then, but I do know that this particular skill goes back as far as I can remember. The stories the puppets at church were reenacting were much more similar to the stories presented as fiction than those presented as fact. So I took them as fiction. As a result, I took the premise behind them (that a god exists) to also be fiction.
Since then, I've never been given any reason to see it as anything else. Because of my reverence for truth and the number of people proclaiming it be true, I've searched for such a reason. I've found reasons people want it to be true. I've found clever wordplay, from people desperate to prove it's true, but none that held up to basic logic tests. I've even found psychological explanations for why someone would believe something even while acknowledging they have no good reason to believe it to be true.
But I've never found any tangible reason to actually believe anything super-natural exists in any form. As a result, I'm still as atheist as the day I was born.