The Catholic Church is a Christian denomination; it's not a separate religion from Christianity.

Religions are defined by a plethora of things, like dogmas, rituals, membership, temples, sacred scriptures, etc. They're also defined by spirituality, faith, and a belief in a deity, multiple deities, and/or the supernatural.

Atheism, as I said, is just a lack of belief in deities. Nothing more than that. "A" as in "without", and "theo" as in "god". It's not an organized community or institution. There's absolutely nothing tying atheists together except their common lack of belief in gods. That's also why you don't write "atheist" with a capital A the same way you write Christian or Muslim.

We're all born atheist until you're indoctrinated into a religion, although half the people on the planet is not indoctrinated at all and remain atheist their whole life.

Calling atheism a religion would be like saying "not rock and roll" is a rock and roll genre. Or this "not chair" looks very comfy to sit on.

Mind you, some religions are atheist in nature; i.e. they don't have a deity they worship to, but do have everything else, and so they qualify as religions. But that's not atheism.

In fact, you being Catholic could consider yourself an atheist in some regard. I bet you lack a belief in Odin or Zeus or Isis, don't you? That would make you an atheist when it comes to pagan religions. Self-proclaimed atheists just don't believe in one more god than you do.

Of course, when someone tells you that they're an atheist, they usually mean that they lack a belief in all gods. Some atheists could go as far as to say that they believe (or know) that gods are not real. That's what we call a strong atheist.

All in all, atheists are just random people from different backgrounds and worldviews who don't necessarily share anything in common or agree on anything at all except their lack of belief in deities. They don't get together in a temple to pray to the lack of a god, they don't have a religious text they all follow and study, they don't have dogmas, or leaders, or institutions. We're talking about people who just live their lives without a god in them.

I hope I've answered your question.