I remember a very distinct episode from my childhood where I was told by someone else about asking God/Jesus to come into your life. I was about 2 or 3 years old, and the whole idea didn't really make any sense, but it was an older kid that was telling me so I just took their word for it. If other people are anything like myself, they would have had a similar experience at a young age of someone else telling them about god, and just accepting it as true, because children are simply gullible and naive. Up until that point I was by definition an atheist. I had no conception of gods or religions. Sure, I had been taken along to church from a very young age, but that is one of the earliest memories I have, I can't really remember anything else prior to that, so if that event had never happened, and if no one else for the rest of my life had ever mentioned gods, religions, spirits, demons, souls, supernatural occurrences and the likes, and if I'd never come across any material vaguely related to them (yes I know it's implausible, just stick with me) then I would still be an atheist today.
So, my whole point is that everyone is essentially born an atheist, even those born into Christian families as I was.