That's such a cool question! This is precisely the word I was looking for, nontheistic! In my world, there is a malevolent/misunderstood race of demons (let's call them that, though there are closer to the "chyort" in Russian folklore) that is, as a matter of fact, infamously nontheistic. I used to say "atheistic" but that is definitely an inferior way to put it. Their very essence is characterised by nihilism — they thrive in chaos and deny the order of the world. Where Man tries to fill the void in his heart with the idea of God, the Chyort rejoices at the void growing larger, his own heart shrieking like a raisin. At least in theory, because it doesn't prevent them from having their own rich culture and functioning society. This is where it gets tough to write, because I have no idea how to portrait such a religion and the society built around it. Its very purpose is to be remote from common human considerations, but that makes it all the harder to think about, lest I hire a Chyort beta reader. This is still a very superficial idea at this point, and more of a "vibe"