Atheism is not a religion. But the following is not a good reason for thinking so:
Atheism (and here I mean the so-called “weak atheism” that does not claim proof that god does not exist), is just the lack of god-belief – nothing more and nothing less. And as someone once said, if atheism is a religion, not collecting stamps is a hobby. That really ought to end the discussion right there. Clearly, a mere lack of belief in something cannot be a religion.
Right, a mere lack of belief in something cannot be a religion. But atheism is not a mere lack of belief in something. If atheism is just the lack of god-belief, then tables and chairs are atheists. For they lack god-belief. Am I being uncharitable?
Suppose someone defines atheism more carefully as lack of god-belief in beings capable of having beliefs. That is still unacceptable. Consider a child who lacks both god-belief and god-disbelief. If lacking god-belief makes him an atheist, then lacking god-disbelief makes him a theist. So he is both, which is absurd.
Obviously, atheism is is not a mere lack of belief, but a definite belief, namely, the belief that the world is godless. Atheism is a claim about the way things are: there is no such thing as the God of Judaism, or the God of Christianity, or the God of Islam, or the gods of the Greek pantheon, or . . . etc. The atheist has a definite belief about the ontological inventory: it does not include God or gods or any reasonable facsimile thereof such as the Plotinian One, etc.
Note also that if you deny that any god exists, then you are denying that the universe is created by God: you are saying something quite positive about the ontological status of the universe, namely, that it does not depend for its existence on a being transcendent of it. And if it does not so depend, then that implies that it exists on its own as a brute fact or that it necessarily exists or that it causes itself to exist. Without getting into all the details here, the point is that if you deny that God exists, this is not just a denial of the existence of a certain being, but implies a positive claim about the ontological status of the universe. What's more, if there is no creator God, then the apparent order of the universe, its apparent designedness, is merely apparent. This is a positive thesis about the nature of the physical universe.
Atheism, then, is not a mere lack of god-belief. For it implies definite positive beliefs about reality as a whole and about the nature and mode of existence of the physical universe.
Why then is atheism not a religion? No good purpose is served by using 'religion' to refer to any set of action-guiding beliefs held with fervor and commitment. For if one talks in that hopelessly loose way, then extreme environmentalism and Communism and leftism are religions.
Although it is not easy to craft a really satisfactory definition of religion, I would say that all and only religions affirm the existence of a transcendent reality, whether of a personal or impersonal nature, contact or community or identification with which is the summum bonum and the ultimate purpose of human existence. For the Abrahamic faiths, Yahweh, God, Allah is the transcendent reality. For Taoism, the Tao. For Hinduism, Brahman. For (Mahayana) Buddhism, the transcendent state of nirvana. Since atheists precisely deny any such transcendent reality, contact with which is our highest good and ultimate purpose, atheism is not a religion.
"But aren't militant atheists very much like certain zealous religionists? Doesn't militant atheism function in their lives much as religion functions in the life of the religiously zealous?" No doubt, but if one thing is like another, that is not to say that the one thing is the other or is a species of the other.
And another thing. If atheism is not a religion, then, while there can be atheist associations, there cannot be, in any serious sense of the word, an atheist church.