[MENTION=6801685]Phantarch[/MENTION] But when we say "Atheist" in the terms of the Realms, are we talking about someone who denies the existence of the gods, or simply someone who denies them worship?
When we say "atheist" in the realms, I would say we are talking mostly about people who deny the
divinity of the gods, not the existence. Denying their existence is tantamount to utter insanity. For the sake of this discussion, I would argue that "divinity" (what actually makes a god a god) is being the embodiment of an aspect of nature (be it physical, ideological, or whatever) and being a necessary component of it. An atheist would argue that the gods being necessary to the continued functioning of the world is a false assertion (e.g., Gravity will continue to exist even without Gravitron, Lord of Mass).
I would argue that "Faithless" would be anyone who denies worship to the gods. So, Atheists would be Faithless, but not all Faithless are necessarily atheists.
As it pertains to the Wall, I think if there was a person who chose not to worship the gods, but was otherwise good in every other way...defended the helpless, fought against evil and tyranny, actively strove to make the world a better place...and a good deity allowed that person to suffer for eternity in the Wall of the Faithless, then I would say that deity has justified that person's stance that the gods are not worthy of worship.
I would assert that to ask whether the gods are deserving or worthy of worship is the incorrect question. The question is whether worship is NECESSARY. The Faithful would argue that it is (and indeed, the gods would as well). You don't worship Tyr because he's a great guy, you worship him because without him, the ideal of justice would cease to exist. If you believe in and fight for Justice, then by extension you would believe in and worship Tyr. Acting against the gods that are the embodiment of your personal ideals is self-defeating.
Additionally, regarding the Wall, the defense you are using is akin to saying that if a person decides to be a vigilante, steals guns and money from the police to do so, and murders every registered sex offender they can find in the name of justice, he should be given a police pension rather than go to jail. That's not the way the system works, and the vigilante is acting outside of the established order, and is therefore subject to punishment.
And I do believe that the gods exert influence on their specific domains or portfolios, but those things don't cease to be when a god dies or is otherwise rendered inactive. Mystra died and Magic went haywire yes....that's one example. But Bane was killed, and tyranny didn't vanish from the world. When Torm died, paladin's didn't all go with him. In Bane's case, his portfolio was taken over by the mortal Cyric. Torm was shortly restored by Ao.
Again, as I said, I don't have a firm enough grasp on Realms lore to know the nuances of every god that has died, but I believe that typically anytime a god has died, a mortal or another god has taken up the portfolio. When somebody else takes up the portfolio, they become the embodiment of that portfolio, allowing things to continue on as they were. Portfolios changing hands doesn't lead to destruction. However, if the gods were no longer worshipped, there would be no entity to safeguard those portfolios.
There is enough anecdotal evidence from the stories to support the view that the gods are not supremely powerful and deserving of worship as creators of the world and mortal kind. Certainly Kelemvor has influence over the souls of the dead, and he can exert that influence in a number of ways. Does that mean he created mortal life? Or the world? No...until a 150 years ago in FR time he was just a fighter.
Who said anything about needing to be supremely powerful and a creator of the world and mortal kind in order to be deserving of worship? This is a very narrow view of godhood that even in the real world is typically limited to the monotheistic religions. For example, the Greek gods were upstarts who killed or imprisoned their parents, and had nothing to do with the creation of the world or mortal kind.
Also, apotheosis is a common concept in many ancient religions, and such people were worshipped for what they represented and achieved, not what they were born as.
Even that aside, I still maintain that the real problem isn't being deserving of worship, but needing worship. There is a symbiotic relationship between the gods, mortals, and the world in the Forgotten Realms. The gods maintain the world, but need the worship of mortals to do so. Mortals need the world, so they worship the gods to ensure that it exists.
As a final thought, there is still the question of what constitutes worship, and this isn't necessarily laid out anywhere. Personally, I don't think this means showing up to the temple of your patron deity on Sundays and donating 10% of your loot. Worship is probably more in line with living up to the ideals of the gods and representing them in the world. So, when your character is about to overthrow a horrible despot, he says, "Tyr give me strength", not "Screw you Tyr".