Atheists are just people who don’t believe in any God. Catholics are people who don’t believe in nearly all Gods. But they believe in one and that makes them a religion. Atheism, stated differently, is the absence of any religious belief. It might be possible to be an atheist and believe in spirits who are not God, and other spiritual forces but most atheists in my experience reject those ideas too, and would not, say, consider Buddhists atheists despite their lack of a personal deity. In my experience atheists who can be bothered listening to what religious people think about us are mildly annoyed by efforts to present atheism itself as a religion. Most of us are open to seeing religious aspects to some forms of organised atheism, just as we can see religious parallels with tropical fish organisations. People get together and do things in similar ways whatever the cause. It is because we are social animals, evolved to do so.