Yes, Atheism is a Religion
It's really obvious when you think about it: When atheism critiques religion, it inevitably ends up being either religious or very much like a religion.
Take, for instance, what atheism purports to say. Any meaningful form of atheism says either that God doesn't exist, or that one ought not to believe in God. They don't just mean this for themselves ("Oh, I personally don't believe, but it's perfectly okay if you believe"). Instead, they take it that not a single person has a rational basis for believing in God.
These are very strong claims. There's no science to back it up. No empirical evidence for it. So on what basis are these claims made?
There's one clear explanation: Blind faith. Atheists have blind faith in some claims, and hence they form part of an atheist's belief system.
We don't have to stop there. We can look at the definition of religion. Many scholars of world religions don't take religion to be defined as beliefs in God and the supernatural. This rules too many religions out. Instead, they look for one common ingredient in all religions, and that is the state of being ultimately concerned. Having a "most important thing" that you care about. And while, strictly speaking, atheism itself doesn't constitute all of one's religion, the broad pool of beliefs, of which atheism is a necessary and important feature. In that sense, atheism is a necessary part of one's religion. The transference of one's ultimate concern from God, to themselves.
There's a third way in which atheism does the job that religion does. Atheism itself, or as part of a larger worldview, informs our answers to the ultimate questions in life. If you find yourself an atheist, you are NOT ALLOWED to believe some set of answers to questions like "What is the meaning of life?" "Where did all of reality come from?" "What happens after we die?" "Are we more than just our bodies?". As such, atheism not only plays the role of dogma, in defining what we are not allowed to believe. It defines the range of answers we can take to be true.
For these reasons, I think it's OKAY for people to believe atheism is a religion, and for atheism to be treated like a religion, even by atheists.