Athar

The Athar are one of the Factions that contest for the minds & souls of Sigilites in the Planescape setting for Dungeons & Dragons. A weird mixture of atheism and Buddhism, with a touch of monotheism, the Athar are united under the single, simple belief that the various gods of the multiverse are not "real" gods, but rather merely extremely powerful entities who are exploiting the gullible denizens of the planes; they do believe there is some kind of "true" divinity, the Great Unknown, which lies beyond anything that mere mortals can hope to perceive - at least in their present state.

Needless to say, they're not a very popular faction in-universe, for various reasons; the Harmonium likes to use religion to promote conformity and so disapproves of someone attacking the cornerstone of religions, whilst the Mercykillers are leery of accepting the idea of "the Gods are Frauds!" because that'd mean that Justice would demand they be punished, but they're too strong for the Mercykilelrs to stand a chance, for example. About the only faction that really likes them are the Believers of the Source, since their philosophy of "ascend to divinity by unleashing your innermost potential" is fairly close to the Athar's own beliefs.

Knowing they're on shaky ground as perhaps the least-liked faction of all prior to the Faction War, the Athar focus mostly on attempting to score conversions through propaganda, with a particular emphasis on harassing clerics and exploiting religious layfolk suffering crises of faith. After the Faction War, they are forced to flee to the base of the Spire, where they can still be shielded against divine magic now that they've been kicked out of Sigil.