This isn't too different from my own experience, though I didn't come into the formal arguments against God until I was a bit older. Initially, I rejected a lot of what I was being taught in Sunday school because I was already a huge nerd as a kid and reading entirely non-fiction (natural sciences, specifically) a the time. I was patently uninterested in any sort of fiction - I wanted the facts and anything else was rubbish. Naturally, that included everything teachers were attempting to tell me in Sunday school which I perceived as boring, useless stories and fairy tales. Later on I became aware of the more formal arguments against the Christian god, like the problem of evil and the like, which just solidified my "religion is stupid, theism is stupid" angstheist attitude.
But, like you point out here, all of this was really a result of my own narrow-mindedness. When I discovered Paganism was a thing in college, theology was easily the biggest struggle. Like most in Western culture, I was in something of a mental prison where I could only think about "god" in the way that is taught within that culture. Anything else simply wasn't a "real god," which meant I ignored any serious consideration of any theology other than classical monotheism. In discovering Paganism, I started becoming dimly aware of the fact that I was thinking inside a prison. Then dim awareness became revulsion as I recognized my culture had conditioned me into this prison without my awareness. It needed to be slagged, and slag it I did. But that process took a long time.
This experience is part of why I'll remark from time to time about foundational assumptions that underpin our worldviews. We inherit a certain set of foundational assumptions from our environment - the culture we are raised in - and rarely if ever question or challenge them since we aren't consciously aware they exist. This happens routinely with ideas about what "god" means, and is especially chronic since the public education system in my country does nothing to ameliorate this. A child should not be able to go through K-12 education still thinking "god must be supernatural" or "god must be omnipotent." Yet it happens all the time because public schools refuse to teach basic competency about theology for fear of... well, 'nuff said.