How do you logically justify being atheist in a default D&D campaign(not-Planescape either)? Just some straight up plain vanilla Greyhawk or Forgotten Realms where everyone *knows* that there are gods that grant their clerics spells and favor to their followers? What about Krynn? Especially on Krynn, where if everyone knows that if you deny or defy the Gods they can leave or just literally throw a mountain down on your head.
In the Dragonlance setting, Raistlin proved that Belzor was a false god, and when Caramon and Tasslehoff traveled to the future, they discovered that Raistlin had also destroyed Takhisis and all of the other "true" gods. Then there are the barbarian tribes that worship dragons, the Cult of the Worm that worshipped a huge worm, the Temple of the Forerunners who follow their ancestoral spirits, and so on.
I doubt many people would lack belief (a la atheism) in the existence of dragons, ghosts, huge worms, etc. But believing they exist is not the same as believing they are gods. They're more like personality cults, in the same way that certain pharaohs, prophets, kings, queens, heroes and emperors were/are considered deities. Is Takhisis a god? Some would say so, but then many said the same thing about Belzor until Raistlin proved otherwise. And the only reason Takhisis is still alive is that Raistlin chose to spare her life...
Of course it also comes down to definitions. If you use the Oxford Dictionary definition of a god as "a superhuman being or spirit worshipped as having power over nature or human fortunes" then that would cover all sorts of undead and magical beings. In that case atheism wouldn't make sense - just as the concept of faith ("strong belief in the doctrines of a religion, based on spiritual conviction rather than proof") would become redundant - when the proof is all around you.
In the Forgotten Realms setting there was a Time of Troubles where the gods were stripped of their power, and many of them were killed, only to be replaced by former mortals. Once again it's going to be pretty difficult to lack belief in the gods, but I envision it as being more like the Marvel universe; if some guy who used to hang around your local pub suddenly developed superpowers, would you consider him a god and start worshipping him?
I think the Eberron setting is also worth a mention, as it has a rather interesting stance on religion. The Sovereign Host have no interaction with the material plane, and there's no direct evidence of their existence, nor is there an afterlife for the faithful. There are also clerics who worship distinctly non-divine entities (such as a warforged terrorist leader, and a half-dragon lich) and apparently wield magical power as a result, strongly implying that their power comes from within. While there are certainly plenty of magical beings in Eberron, including some (such as powerful dragons, and outsiders such as Radiant Idols) who desire worship, there is nothing that couldn't be defeated by a band of heroic adventurers. I don't imagine many people would worship a creature that is demonstrably weaker than
they are.
But in fantasy settings where immortal gods walk among mortals, a more appropriate stance might be misotheism or maltheism. Even in Norse mythology there were stories of mortals who turned away from the gods in disgust, preferring to trust in their own strength and virtue.
