What is atheism?
If you ask an atheist this question most often they will carefully explain to you that atheism is ‘a lack of belief in God’.
They’re very sensitive about this.
A lack of belief is not the same thing as non-belief. To claim God ‘does not exist’ is an absolute statement, and one which is virtually impossible to prove deductively.
So the atheist will simply say they ‘lack belief’ and will say this is mostly because of a ‘lack of sufficient evidence’ (agnosticism).
Some atheists are happy to flat-out deny the existence of God and explicitly deny He exists, but most do not.
In practise however, virtually all atheists live life as if God does not exist and harbour a characteristic antagonism towards religious belief.