this question is: in a fantasy world (such as Toril) filled with magic and miraculous beings, will the religious concepts of the locals be completely different from those of the human of Earth(which a magicless world)?
In such a world, everything is clear and knowable. You know that various magical powers, gods, and other similar beings exist. you know that after you die, your soul will go somewhere to enjoy eternal bliss or torment————and more importantly, you can (at least in theory) actually know and verify this, such as through the spell of Planeshift or Gate. although such spells are only available to a few of the most powerful people, they are indeed possible and exist.
In this world, miracles are cheap. If all you can do is walk on water, feed a large crowd with a few loaves of bread, or cure a few diseases, it's nothing. while not everywhere, every larger town usually has one or two people who can do it. more importantly, even if you can perform somg greater miracle, like make people resurrection from the dead, it doesn't mean much. raising the dead is a powerful spell, but there are countless powerful people who can cast it————how can you prove that you are superior to others?
In a fantasy world where magic and countless miraculous beings actually exist, people even the lowest peasants won't believe your empty words; they demand proof, tangible power. in other words, according to Earth's concepts, they are actually MATERIALISTS AND ATHEISTS————they know that God exists and worship them, but their concepts of such things is completely different from that of magicless Earthlings. simply put,unlike our Earth,in the fantasy world,if a missionary attempts to spread faith with empty words without demonstrating any real miracles, no one will care them.
Well, first, they wouldn't be materialist atheists--they'd be gnostic theists; they both believe in the gods and know they exist.
I would assume that the difference between mortal and god is scale. A human can (with a big enough diamond and a long enough ritual) raise the dead. A god should be able to raise multiple dead without any material components and at a moment's notice.
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Should be, of course, is the key--D&D loves to stat up their gods, or at least their gods' avatars, and thus limit them to what the spells can do. And, of course, real-world gods often didn't have that level of power anyway, or they relied on magic items to do the cool stuff.)
The thing is, the average person isn't likely to see all that much magic--enough to know it exists, enough to see it be used for impressive things,
but not enough to actually know how it works or its limitations. How many 5,000 gp diamonds exist?* How many clerics are high enough in level to cast it? Heck, how many clerics are high enough level to cast it
and have big enough diamonds laying around? Even if an entire village or town chipped in, it's unlikely they could afford to get
anyone raised, even if the local cleric wasn't charging for the casting. They're not going to see a
raise dead in action. They're probably never even going to see a
revivify. At most, they might see a
spare the dying, if they're lucky enough to have both the dying person and the cleric in the same room together.
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* Can you just go grab any old diamond, pay 5k for it, and it's good enough? They should have specified carats, like GURPS does for its mana stones.
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At best bet, the average person is going to see injuries and illnesses get magically treated, receive some relatively minor blessings, and maybe some heavenly (but bland-tasting) manna made during starvation situations. They're not going to see people walking on water simply because that's not something that is going to be needed in their lives, meaning there's no reason for the clerics to cast it. Ditto for arcane casters. Unless they live in a monster-infested place (which is possible), the wizard is going to be doing mostly minor tricks for entertainment and utility spells.
So if you have some larger-than-life figure come in and do amazing things, there's really no reason why people might not accept them as a god.
(Unless, of course, this is a setting where the actual gods get ticked off at mortals who do this and either intervene with their clerics and paladins or go straight to the smiting or cursing.)
Sure, there could be a demand for proof. But it wouldn't necessarily take big miracles. The Greek gods didn't bleed blood, they bled ethereal but highly toxic ichor. There's no reason why a fantasy god couldn't have something similar. You claim you're a god? Here's a knife, go shed a few drops. We'll even sacrifice a chicken you can bleed onto. If it dies a horrible and clearly supernatural death, you're a god. That won't stop really dedicated fakers, but it will stop most of them.
