If it doesn't appeal to usually invisible, mostly incomprehensible forces beyond the natural world, and claim that earthly reality is less important than what happens on or through the operation of some other plane of existence, it isn't a religion.

No amount of special pleading affects that basic premise. Every culture on earth recognizes it. The Communists who banned religion could obviously tell the difference, even if some people have convinced themselves there is none.

Atheism is not a religion. Agnosticism is not a religion. A sociopolitical system is not a religion. Britney Spears is not a religion. Aztec cosmic paranoia was a religion.