Not that this is the subject matter, but it is a religion and it is based on the “belief” that there is no God/god/gods. It is the definition for atheism and based on faith since one does not know if it is true or not. Atheism defined:
- The doctrine that there is no God; denial of the existence of God.
That's not the definition that most self-described atheists use. It's not mine. It would exclude somebody like me, an agnostic atheist, who does not assert that gods don't exist.
Yours is a definition Abrahamic theists like, because it allows them to present an artificially low estimate of the number of people in the world with no god belief. The rise of the nones and the increasing social acceptability of atheism along with a concomitant fall in the relevance of religion in daily life results in positive feedback accelerating the trend toward irreligiosity, which threatens to organized religion.
If we can assign the meaning of saying that gods don't exist to disbelief, and unbelief to mean a lack of a god belief, then we can see that the former is a subset of the latter and it becomes redundant to specify the subset. And that's not the only redundancy in the OED definition. "God" is a subset of gods. The succinct form of the definition of atheist is the absence of belief in gods, but I know that many theists like to cavil about that definition because it includes people who have never thought about gods, prelinguistic infants and children, dogs, and even stone. For them, we can expand the definition of an atheist to anyone who answers no to the question of whether they hold a god belief.
To expand on that attempt by the faithful to try to present atheism as anomalous and atheists as marginalized by defining atheist in the narrowest possible way, I believe it's related to why you call atheism a religion and to the rise of creation pseudoscience. They're both attempts to put science and religion on equal footing first to try to get gods into science classes ("Let the kids decide"), and also to comfort creationists who are troubled by the differences between science, which has been wildly successful at discerning how nature works and improving the human condition and creationism, which offers guesses that would be useless even if correct. What useful ideas have followed from creationism? None. So, the apologists want a bit more science on their side and a bit more religion on the other.