There are three main paths to atheism, having never been indoctrinated and valuing evidence and reason above faith and belief is one, having been indoctrinated but deciding eventually that you value evidence and reason is another and finally, having been indoctrinated before becoming atheist out of pure rebellion. Not definitive and combinations are possible but I think it's close enough.
You've excluded the most important path which accounts for the overwhelming majority of atheists - growing up in an environment in which most other people were atheists and going along with that. The same way almost everyone else acquires their religious beliefs.
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