Being an atheist means being aware that God is only a product of the brain. This is formed from the primal structure of the hereditary mass, which emerged in the beginning of human life - millions of years ago - to explain the world to itself. (The brain tries to explain everything when it has to deal with something - and you always have to deal with the world. To do this, it is usually not essential to dig deeper into the subject; a simple explanation is sufficient, which is plausible for the brain is.)
To be an atheist also means to be aware that there are only substances that work according to the law - and that a god cannot change this!
As long as there have been people, they have designed the world to deal with it. And the less they knew, the more the brain balanced it with fantasies.
One more note: In the narrower sense, an agnostic is of course not an atheist because he is in doubt as to whether there might be God.
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