Rules Tinkerer and Freelance Writer
Devoted to a concept? Sure! I was just saying the same thing Jmarkdr2 says in the first part of his reply:If Warlock is deemed just a pile of mechanics for arcane character concepts, completely divorced from making a pact with some kind of intelligent (or alien) being (ie, drawing power from an ideology, an organization, a personal belief etc...) then why couldn't a character concept who is devoted to not-religion draw power from an ideal such as the "Cosmic Balance" or "Mathematics drive reality" or "the Multiverse" etc... I mean... could an uber Philosopher or Buddhist monk use the Cleric class mechanics? Why not?
PErhaps another take: Clerics devoted to the Philosophical concept of Chaos, Law, Neutrality, Karma, Entropy etc...
A true "Atheist" would be a person in denial of objective reality. Now an -Anti-Theist-, like most of the modern famous atheists, -that- would work just fine."There are no gods" atheists are the flat-earthers of most DnD settings. "The gods are stupid and no one should worship them because they're stupid" is more in-line with modern atheism (which tends to not only reject but critique religion as a concept) - and a pretty reasonable point of view in a lot of settings.
Not sure I'd play them as a cleric, though. If they wanted that particular spell list we'd come up with something.
An actual Atheist in Forgotten Realms is like someone who screams "THE SKY IS BLUE!" on Athas. Ignorant of reality or delusional.
Though in FR atheist souls get turned into bricks, in the afterlife, to build the city of the dead with. So that's at least interesting? The "Sky is Blue" guy probably just gets eaten by a Thri-Kreen or an Elf.