In a fantasy world filled with magic and miraculous beings, will the religious concepts of the locals be completely different from the human of Earth?

[Sepulchrave II]

Let's assume the following scinario:

In a fantasy world like Faerun, two gods engage in war and command their mortal worshipper to fight each other.

In the end, this war ended with the death of one god and the victory of another, and the worshippers of the loser either converted to other gods or accepted reality, pledged allegiance to the conqueror who killed their god and accepted him as their new god. Only a very handful of most fanatical worshippers would continue to fight, and usually their fate would be very tragic.

So, what is the essential difference between the war between these two gods and the war between two mundane kings on Earth?

It really depends on your divine ontology.

Maybe if the war god is killed, people can no longer commit acts of violence. Maybe if the Sun god is killed, the world is plunged into darkness. Maybe if the storm god is killed, it stops raining etc.

Maybe another deity will step into the deceased god’s place. Maybe the triumphant deity will absorb the portfolio of the dead god. Maybe some other metaphysical remedy is necessary.

Maybe the god can only be killed on its home plane. Maybe the God was killed on its own plane.

D&D in its various iterations has tried to address some of these ideas. The mythic logic of the campaign setting may offer other ideas. But without knowing what that mythic logic actually is, then there really isn’t a single answer.

If the mythic logic is “Gods are just like very high-level characters,” then treating them as such would be consistent.