Henry said:
a mortal who actually didn't believe in the supernatural at all in a D&D world is silly
It's interesting, because in a D&D world, the supernatural...isn't.

It doesn't take a lot of "faith" or "ignorance" to believe in fairies and magic. It's a concrete reality that affects your daily life. The "gods" fall into this category, too. It requires no belief, no faith, to accept it as real. It would be like not believing in electricity, or gravity, or climate change, or evolution, or something. It's clearly a thing that is.
So in a D&D world, not even clerics have what we might call "faith." Divine magic is a route to power like any other kind of magic. They don't need to believe, they just need to obey, worship, etc.
There's nothing, metaphysically, in a D&D, that people believe that might be doubted. Similarly, since the metaphysical is concrete, you don't have to express faith in its veracity.
A "distant gods" setting (such as Eberron) gets a little closer to the world as it actually works, but even there, miracles are known to happen to the faithful. The
source of them is just more murky.
Of course, atheism isn't necessarily incompatible with the supernatural. You can believe in ghosts and aliens and psychic energy and whatever else without necessarily believing in a god.
Kind of hinges on what a "god" is, I guess. In D&D, a god is mostly just a creature who can give magic to those they choose. The distinguishing feature is that personification. Perhaps D&D atheists don't believe the personification is valid -- that there's divine magic, just as there's arcane magic, but it doesn't need to be mediated through a divinity.