I never learned to believe.
My grandparents took me to church Sunday School (to give my mom a break, she was very ill when I was little), and later went to Sunday School on my own when my mom moved elsewhere, until I was about 10. I was really into setting up the Christmas creche decorations in my grandparents' house. However, all along, I thought it was storytime, like library read-aloud stuff my mom took me to, with arts and crafts. I was really into fantasy, including The Hobbit and Wonder Woman and such (it was the 70s, and there were lots of kids level fantasy stuff in books and on TV), and I didn't know people actually thought the Bible stories were REAL. I figured it was a social play-along game, like Santa Claus, Tooth Fairy, or the Easter Bunny. It made as much sense.
I was about 10 when the church I was going to at the time decided I was old enough to attend services instead of Sunday School classes. It was a rude awakening when I realized the *adults* around me thought it was all real. I laughed at first, I thought it was a joke. Then I was horrified. Then I thought it was a tiny minority of freaks, like the Flat Earthers. I quit going to church right then and there, though I missed the social parts of it.
It wasn't until I was about 16 that I first heard the word "atheist" and I was still pretty meh about it. I still thought non-believers were the majority, it made no sense to me that reasonable, rational minded people would *believe* such crap. It wasn't until I was about 22 that I actually became aware that the *majority* of Americans were believers.