Raven Crowking said:
In a fantasy world, gods are real beyond the shadow of doubt.
No, they're not.
Not even the majority of PC's ever meet a god; the great masses of NPC's most certainly don't.
From a meta-game perspective, the player's and the DM may know beyond a shadow of a doubt that gods are real, but the vast majority of characters in the game world don't.
And those characters that do? They're right up there with Mohammed, Abraham, and any other religious figure you might mention. Many believe what they've said about religion and the after-life. Many don't. For all anyone knows, they may have met or spoken with their deity. But, just like the vast majority of NPC's, they don't have anything but faith and trust to go on.
The circumstances for worship for the vast majority of people in a game world are just the same as they are for those in the real world. Most people only have what they're told to go off of.
Sure, there are more fantastical elements, depending on how you go about things, but none prove anything more than another. Being able to cast clerical spells without a god certainly isn't going to sway everyone; if anything, those powers might be seen as coming from an infernal source, and wind up getting the god-less cleric burned at the stake.
And one thing to keep in mind is upbringing and local authority. This is one of the biggest factors in faith; what a child was raised to believe in, and what the local ruler says is the religion or not.
If you're raised to believe in The Grand Pantheon of the Ineffable from birth, what reason have you to question it? Even if you run across information that runs contrary to your doctrine of faith later, so? Most real religions have information that runs contrary to their teaching - information that sometimes is better backed annd supported than the religious information. Yet people continue to believe in what they were brought up to believe, despite all evidence that says otherwise.
And if Bob, God of the Ineffable, appears before the king in a burst of smoke and fire, and says "I'll give you goodies if you spread my word," well, than the people will follow when the king starts to worship. Without much more divine intervention than that. And once the tradition is established, it'll perpetuate itself easily enough (at the very least, it will so long as there's no other prevailing religion at the time).
And if there's one thing nebulous concepts can't do, it's enforce their will. It's come down and offer messages to people. It can't let its presence be known.
It's also less easily conceived of than a solid figure of some sort. Strength as a concept doesn't exactly provoke a strong, defined image. Sure, a number of images can be visualized - but Strength isn't quite so strong as, say, Thor, the redbearded, brawny god of smiths and justice, protector of the common man and slayer of giants and serpents. It's harder to make morals, parables, and stories about abstract concepts; or less interesting, at least.
Oh, not to mention that the cleric, while never implicitly mentioned as needing faith, implies as much. And faith comes first - not power. I don't allow clerics or the like into my games who don't have faith. They're granted their power thanks to the strength of their belief; their belief does not come thanks to the power they're granted.
And as another aside, generic D&D is meant to be a building block. Most campaign settings ignore aspects of the base D&D, or change it, or specify it where it's left ambiguous.