Okay for @GregPogorzelski and others, the story of Why Clerics Are in D&D.

The first thing you need to realise is that before D&D there were no clerics or priests in fantasy. The sole except is Friar Tuck. In fantasy, religion is almost entirely EVIL. Like in Conan with cultists

A key way that fantasy says "we're not just medieval europe" is by NOT HAVING A CHRISTIAN CHURCH OR ANYTHING LIKE IT. Even Warhammer specifically calls its religions cults and gives them temples. See also Runequest. No priests, plenty of cults.

So why is the priestly christian type cleric in D&D? The answer is Lord Fang.

Gygax is playing an all-evil campaign. Now thing about Gygax and his groups is that they are EXTREMELY antagonistic. Everyone is trying to mess with everyone. One guy makes a vampire called Lord Fang and he is unstoppable.

Nobody can kill Lord Fang because he just turns into fog or bats and shit. So Gygax solves this the way he solves everything in D&D: instead of balancing the game, just adding more antagonistic shit. And he looks around and goes "What kills vampires?"

It's 1974. What kills vampires? There's only one answer:
https://resources.stuff.co.nz/content/dam/images/1/y/p/n/u/k/image.related.StuffLandscapeSixteenByNine.1420x800.1yp325.png/1579028781993.jpg

PETER CUSHING.

Enter the Cleric, who has the power that Peter Cushing has in Hammer Horror films: he can pull out his cross and make the vampire recoil. It gets called TURN UNDEAD.

Then they realise that clerics need more than just that power so they gave them healing magic as well

At no point does anyone add a church or a religion properly. They just added Peter Cushing with healing. Slowly, every other thing is glommed on and tried to make meaningful, AFTER THE FACT.

Clerics don't belong in fantasy games. It's FUCKING WEIRD.

Because they're only there because of Peter Cushing. And that's what I mean when I say D&D isn't a great game. It wasn't really designed it was oozed out at as a strange conglomerate and then back-justified. And then, frighteningly, accepted as "the way it is"....

ANyway.

The end