A standard rejoinder is that the non-existence of a deity is unprovable, so an affirmative disbelief in one is also a religion. One can find a response to this argument at this page.
A standard response is to note that if atheism is a religion, then "bald" is a hair color, and not collecting stamps is a hobby, not kicking a kitten is a form of animal abuse and so on. Another is to note that if the definition of religion was expanded enough to legitimately include atheism - say, by defining a religion as "any philosophy on life" - then practically everything in the world would be a religion, such as socio-economic policies or views on equality. (British law has come close to finding this in employment discrimination cases.)
So to answer the question of whether atheism is a religion, we need to have a definition of "religion". OK, that makes sense to me. So I went looking for one, and found one at this page.
Religion is an organized collection of beliefs, cultural systems, and world views that relate humanity to an order of existence.
So if we accept this definition, which would not apply to atheism? The only one I see might be the requirement to be organised. But then, we have organisations like this one.
So have this and other such organisations turned atheism into a religion, by fulfilling the final requirement, that it be organised?