Atheism can be agnostic as in "I don't know if there is a God", because that is a lack of belief.
Atheism can also be a belief, as in "I believe there are no gods or God."
Capital "G" God is The Ultimate Reality. Atheism toward this God is nihilism.
An atheist would say they have no gods, but from at least the orthodox perspective, if you don't accept God, the void has to be filled with some god. The atheist is simply someone who is ignorant concerning what is meant by these things.
The philosophers of the 1800s understood atheism better. In that, when you reject The Ultimate Reality, or absolute Truth, the void is filled. One becomes their own god.
What motivates a person? What makes life worth living for them? What is life for a person? If you examine these things, you will find what god or gods it is that a person has.
Those who in their pride make themselves out to be their own god do not tend to recognize how their preferences reveal which gods have control over them.
The Ultimate Reality is God. That being the case, whether someone acknowledges this God or not, it is the case that this is God. A rejection of this God is an embracing of arbitrariness, a submission to polytheism.
Atheists and polytheists are the same to us, the only difference being one of acknowledgement. The polytheist recognizes their gods, while the atheist is in denial or ignorance of their gods.