SeekAndFind merely copied the title from the source, as per FR rules. A-theism is indeed not a religion proper, but i would postulate that it is arguably basically an ideology in form, based upon the belief that there is no deity, and thus (typically) no supreme universal objective, transcendent moral standard, with morality being based upon what seems reasonable to the atheist, and with the origin of the universe and miracles being attributed to naturalistic causes.
And thus there is the militant atheistic movement in the West, exhibiting hostility toward religion in general, and against the idea of a absolute moral law and a supreme Judge, and traditional support of religious faith by the government.
Which does not have a home on FR, and as it rejects any Divine supreme universal moral standard, it easily lends itself deifying man, in which such a work as Mao's little red book can serve as a Bible, and atheists can advocate the removal of children from the homes of Christians under the premise that it constitutes child abuse, while at the same time advocating such things as fornication.
This does not mean atheists cannot be relatively moral (and the Bible teaches pagans can be so, due to innate moral sense), but there is no universal standard that they can be held to, or even interpreted as being that supreme standard .
Christians of course are also 99% atheists, rejecting all the gods and divine revelation/scriptures books that atheism also rejects saved for one, even though atheists typically treat them as the same.
From Wikipedia:
“A religion is an organized collection of beliefs, cultural systems,and world views that relate humanity to an order of existence.”
So while it’s true atheism and theism are opposites with respect to formal belief in one or more deities, it is not correct to conflate that with religion per se. They are overlapping but different conceptual domains. You have religion if you have any beliefs at all about the purpose and nature of existence.
Furthermore, as Daniel says, these beliefs will condition your choice-making process. They will define you. They are therefore as all-encompassing as any other identifiable religious system. I hear tell some atheists are even starting their own “churches.” That is a telling development.