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?

[Maxperson]

Apollo, Poseidon, and the other top tier Olympians explicitly did not create the world, they just currently rule it from Mount Olympus. Grandma/Great-Grandma Gaia predates them.

All the titans, really.

 

[Voadam]

If sacrificing to a protective god of your city unerringly caused a wall of fire to surround it when enemies attack, I'd say the net effect would be that people would participate in the rituals with even more enthusiasm.

But sacrificing to the gods in D&D does not unerringly cause a wall of fire to surround your city when enemies attack. The closest would be high level clerics having consistent powerful magic.

How the gods interact with people in D&D varies greatly, often there is a don't act directly on the mortal plane agreement. Other times it is like the Greek Myths who might show up in the flesh, but the gods are erratic and you pray to try to keep on their good sides and avoid there bad sides but there is no guarantee.

Most D&D the gods are not doing constant personal miracles to gain your worship.