Atheism is simply the lack of belief in a god. That’s it. “Theism” is the belief in a god(s) and with the prefix “a”, which means “no”’, you get atheism: no belief in a god(s). Atheism makes no claims, it contains no creeds, and provides no guidelines to base worldviews on. No wars have been fought over the lack of belief in a god and atheism is no more a religion than not collecting stamps is a hobby. An atheist is not a person who denies, hates, or rejects a god. An atheist is simply a person who does not believe in the existence of a god.
Gnosticism is the having of knowledge in reference to a god(s) and its existence. With the prefix ‘a’ which means ‘no’ you get agnosticism, no knowledge about a god(s). Agnosticism is about knowledge and atheism is about belief. All honest agnostics are also atheists because one who has no knowledge of a god, has no reason to believe in a god.