There's a difference between a completely fictional deity that exists as a literary device (), and one that has real-world flesh-and-blood worshippers.
Many, or most, of the various historic pantheons that D&D has used for games have real world adherents and are real-world religions as well as being gaming setting elements.
The Japanese Pantheon? That's called Shinto.
The Vedic Pantheon? That's called Hinduism.
The Chinese Pantheon? That's Chinese Folk Religion, also known as Shenism.
The Norse Pantheon? That's called Asatru, also known as Heathernry.
The Olympian Pantheon? That's called Hellenism.
The Celtic Pantheon? That's Celtic Paganism.
Saint Cuthbert, the deity of common sense, truth, and wisdom from Greyhawk, is the same individual of St. Cuthbert of Lindisfarne, a 7th century Christian Saint (venerated in Catholic, Anglican and Orthodox Christianity).
. . .and real-world Satanists have used a number of the various names thrown about in D&D for names of demons and devils in their rituals and worship as well.