but the definition of atheist is:
"One who disbelieves or denies the existence of a God, or supreme intelligent Being."
and disbelieve doesnt simply mean a lack of belief:
"To refuse to believe in; reject."
to be an atheist one must at least have the mental capacity to reject or deny a belief in anything that isnt wordly. or lets say they live in a society that is atheist, so there is no religion to reject. they still have the unavoidable question of existence which is characteristic of any being with an evolved or possibly deluded sense of self. no intellectually functional human can avoid it.
you must first be capable of believing in religion and not believe in it. even with a lack of exposure to religion, you must consciously believe that there is nothing more.