It’s not uncommon to hear some people characterize atheism as a religion. Is it? It depends, in part, on how one defines religion. That’s a tricky subject that does not have widespread agreement. Be that as it may, I think most people would say atheism can’t be categorized as a religion because religions worship deities, and atheism explicitly denies the existence of any deity. However, that’s not accurate. Most forms of Buddhism could be categorized as atheist or agnostic. There is no worship of any deity, and yet Buddhism is properly categorized as a religion.
That said, I don’t think atheism should be categorized as a religion. There are no “cult” practices, for example. There is no set of moral principles that atheists are bound to. Atheism is just an answer to a single question: Does God exist? A point of view on a single issue does not constitute a religion. It is not robust enough. And while it’s true that this question is properly categorized as a religious question, that doesn’t mean that every answer is a religious answer or constitutes a religion. Atheists answer the question in the negative. They do not think God exists. So atheism is a negative claim about what does not exist. How can a claim about what does not exist constitute a religion? Surely religions make positive claims about what does exist, not negative claims about what does not. As a meme I once saw expressed, if atheism is a religion, then “off” is a TV channel.