DocMoriartty said:
One could easily argue though that the gods are merely extremely powerful beings who can be wrong and can be killed, thus are not really gods.
Spoken like someone immersed in the modern monotheistic tradition.
As others have mentioned, infallability and unkillability are not generally traits of gods in pagan religions. The Norse gods could certainly be killed - that's what Ragnarok is all about. And the Greek gods are hardly infallible - the whole Trojan War is based upon divine vanity.
For fantasy worlds, I don't think that disbelief n the gods is like disbelief in kings. The thing that cannot be denied is the existance of
power. Divine spells and powers exist, and the power for them comes from somewhere - but the "fact" that they come from gods is generally hearsay. The observation is that the use of these powers is tied to belief and/or dedication. There's generally no direct observation of the power coming from a deity - which is cool, because that still allows you play with faith-based stories.
If you play strictly by the PHB, in fact, there's little proof of the existence of gods, because druids, clerics, paladins and rangers
don't need to follow a god. Most DMs require it, it's standard operating procedure in most settings to follow one of the setting's gods, but the PHB doesn't require it
If the system doesn't require the existence of gods, then the game must allow for the non-existance of those gods, and thus some form of non-belief is possible.
An "atheist" may think the power is more akin to psi, or another form of the same power as arcane magic (especialy since arcane and divine casters often produce the ecaxt same results). Or, perhaps it's enough to have strong belief, but by and large most people frame that belief in terms of thise god icons (it's easier to have a handle on "Pelor" as a personality than on "Sun and Strength" as ideas alone).