Atheism means simply lack of belief in theism. Agnostics can be atheists as well.
There is no proof that can be determined one way or another. "You can't prove a negative" is a logical fallacy.

The idea that atheists agree on any particular idea is nonsense. They all have different reasons for their atheism and none of those could be said to be religious in any sense.

There is the so-called institutional atheism of Communism practiced by Stalin but this runs counter to the free thought concept of atheism. Stalin, himself, had liturgical beginnings and attempted to make a religion of Communism. It couldn't and didn't work.


Dictionary definition
atheism
noun
nonbelief, disbelief, unbelief, irreligion, skepticism, doubt, agnosticism; nihilism.


So you see there are different forms, not a monolithic view as there is in religion
since in that there is a belief in a supreme being or a god.

The acceptance of any form of religion is not atheism.

Once again, the enablers of the concept of religion can't accept the idea that it doesn't have to exist.