Definition: God is a form contained in the primordial structure of man with the feeling of almighty qualities of a being.
This shapes the brain especially between the 3rd and 5th year of life during human development, in the mystical phase.
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!
Definition: 'Substances' are elementary particles, structures, atoms, neutrinos, molecules, neurons, crystals, liquids, gases, cells, living things, brains, forces, facts, things, living things, ideas, ecosystems, stars, star systems, galaxies etc. in addition, they are also considered to be virtual substances in the states of empty spaces (the vacuum in space).
Everything in the content of the universe is considered substance: in the large, like galaxies - which are relatively tiny in relation to infinity - or in the small, like elementary particles. And of course, everything in humans.
'Law' means that identical parts - or waves - always result in identical structures under identical circumstances.
Laws are properties of substances that, viewed in isolation, are immutable unless something is added or removed from the substances.
And that means that everything can only take place in a very specific legal form.
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.