One of the easiest pieces of anti-atheist rhetoric to come by: "Atheism is just another religion". It's a very clichéd argument. I can't tell if people are just repeating arguments they've heard before, or if people independently come up with it. I wonder if people think themselves terribly clever, essentially arguing, "I know you are, but what am I?" Since it's so common, atheists have come up with a bunch of silly counter-clichés. "Atheism is a religion like bald is a hair color." "Atheism is a religion like not collecting stamps is a hobby." I hate these. It's not so much that I don't like clichés. I hate cliché arguments, because they suggest that the person has not really thought about the inner mechanics of the argument.
"Atheism is just another religion" is just so overwhelmingly wrong, it's hard to counter one stupidity without implicitly accepting another stupidity. I could start with, "So what if it's a religion?" but that would nearly suggest that I think atheism is a religion and that's okay. I could start with, "Atheism is totally not a religion," but that would nearly suggest that I think all religion is automatically bad.
I'll start with "So what if it's a religion?" Just imagine if atheists argued that Christianity is wrong because "it's just another religion." Or if we argued that Buddhism is wrong because, "it's just another religion." We would be immediately and justifiably dismissed. No, the arguments we make are about how the beliefs are either contrary to reality or contrary to good reasoning. These beliefs, no matter how metaphorical or immaterial they are made out to be, often lead to real harm. We can also talk about specific instances of corruption or abuse within religious organizations, such as the Catholic Church's failure to punish sex abuse cases in their priesthood.
But while I think many religions are bad, that does not mean I think that all religions need to be bad. Just because you have a belief doesn't mean it is wrong, nor does it mean that it will lead to harm. Religious communities need not have scandal. It depends on the community. In fact, I'm a social person, I think communities are a good thing, possibly one of religion's only redeeming features.
As a real-world example of a religion I have no problem with, take the Unitarian-Universalist church. Unitarian-Universalists have diverse beliefs, and I probably disagree with a lot of individuals, but I have no dispute with the group as a whole. Note that some Unitarian-Universalists are atheists. So you could in fact say that those atheists have a religion. But their religion is not atheism, it's Unitarian-Universalism.
I'm not really offended at the suggestion that I might have a religion. So what? But at the same time, I find the argument offensive in precisely the same sense that profound ignorance is offensive. Because it's just factually wrong.
When I took an intro course on the history of religion some time ago, I learned that religion is actually a very tricky thing to define. How do you construct a definition that includes everything from Judaism to Taoism, but excludes things like football? But pretty much no matter how you slice it, atheism is hardly a religion. The mere lack of belief in a god is too meager to be a belief system in itself. Even if you consider naturalism, a larger belief system, it's hardly what I'd call a guidebook to your daily life. Humanism might be a little closer to qualifying, but that's another story.
The other major component of religion, an organized community, is also rather lacking. There are atheist organizations to be sure, but I would call it more of a movement than a community. Another interesting thing to note is that before atheism achieved its current level of visibility, many atheists figured out atheism independently, without contact with any community. You could have a single atheist in a small village who has never met or heard of a single person like themself. I don't think this is true of any religion.
But of course, there's much more to religion than just a belief system and a community. There's also ritual, spirituality, giving yourself up to some divine being who loves you. I briefly discussed different ways of being religious over a year ago. Atheists tend to focus mostly on the beliefs and communities of religions, because those are the most deserving of criticism. But I feel very indifferent towards such religious things as ritual and spirituality. Some atheists may seek these out, but it's rarely atheism itself which provides it. And that's fine, because atheism is not a religion and need not fulfill every function of religion all on its own.
But I think a lot of people just don't care about serious definitions of religion. They just want to say that atheism is a religion in a much weaker sense. They just want to say that atheists believe things! And they have groups who like to do stuff! So what? I don't think it's necessarily bad to literally be religious, why would I worry about metaphorically being religious?
I will conclude by noting that atheism is in fact a category of views on religion. Atheists believe that theistic religions are incorrect. Therefore, if someone asks for my views on religion, I can say "atheist". When it's said that atheism is a religion for legal purposes, I take that to mean that atheism is another category of views on religion, equal to any other under the law. Our laws provide certain rights to people of all different views on religion, so I expect those rights to extend to atheists too. I do not believe that this means that atheism is actually a religion.