And compensation, a price in gold, was settled for the Geat Grendel had cruelly killed earlier— as he would have killed more, had not mindful God and one man's daring prevented that doom.
-Beowulf, trans. Seamus Heaney
Paging through the AD&D corpus and thinking about what it all has to say about religion, one is struck by two things. The first is the very high degree of syncretism on display. The creators invented many of their own species of (usually polytheistic) religious belief, and these are mixed in with real-world religions that are typically dead (so that, for instance, the Outer Planes are thought to be home to a variety of 'pantheons' such as the Greek, Chinese, Babylonian, Finnish, Egyptian and so on). And at the same time, of course, individual DMs who invent their own campaign settings merrily create their own systems of religion to sit alongside all of these others, also. If one stops to really think about it, this is suggestive of a vast galaxy of unrelated religions all existing together, and none of which being Truer than any of the others or being able to make a plausible universal Truth claim.
From a theological perspective the oddness of this is breezily waved aside in the source material - why wouldn't there be Finnish and orcish gods living alongside one another in the multiverse? - and, phenomenologically, the ordinary inhabitants of that multiverse see no nice distinctions: gods are gods and exert power in basically the same way, by granting spells to clerics and so on and so forth. And obviously at ground level most people, who never stray more than ten miles from their birthplace, have no conception of any of this anyway - their religion is their religion and that's that. Maybe they have some dim awareness that the local goblin tribe worships some specific deity which is different to their own, and maybe they even recognise that deity to have some real-world power and influence (and might even come to adopt it as their own if it is revealed to have more power and influence than the god their ancestors traditionally worship). But they're not worried about how it is supposed to all make sense.
The other thing that strikes one, however, is the shoe that doesn't drop - there is no explicit mention of Christianity (or Islam, Judaism, Hinduism, or any other modern living religion) in canonical AD&D, for all that the religion is clearly latent within it. The cleric, who banishes undead and splashed around holy water while waving a holy symbol, obviously derives from basically Christian motifs and stereotypes about exorcists and inquisitors; many of the monsters - especially the undead - only really make sense as monsters when founded on a basis of Christian folk belief (which holds that coming back from the dead is intrinsically evil, as opposed to something that happens once a year when the ancestors come home for dinner, or whatever); the devils and demons clearly use the iconography of medieval Christian ideas about hell; and so on. It is almost as though there is an implied Christianity in the typical D&D world lurking in the background, (one could widen this out and say an implied Abrahamic faith), which is unable to fully express itself but which is hinted at at every turn.
I know very well why the creators of AD&D never inserted Jehovah, or Allah, or whatever, into their fictional multiverse - they didn't want to piss people off. But given the standard approach to theological matters (basically that any and every religion that one could think of can exist and has actual divine power), and taking that approach on its own terms, then surely the God of the Abrahamic faiths must also be subject to the same rationale on a 'sauce for the gander' basis. There is not, I mean to say, any principled reason as to why the Finnish or orcish (or whatever) god are 'real' within AD&D world, but the Christian God is not.
Following through on this idea would have interesting ramifications. First, it opens the door to a Beowulf-inflected syncretism, in which old pagan beliefs and pagan demons (Grendel, the dragon, the eotenas, the orcneas, etc.) exist, but alongside a Christianity which is somehow more True, or at least stands in opposition to it, Here, all of the furniture of AD&D would be as it is, but there would be some notion that it is set against an underlying theology which is of a different substance entirely - there is a God who is simply more good or indeed more powerful (for all that perhaps he refrains from acting, for mysterious reasons) than the rest.
And second, it could give rise to a campaign style that would feel as though it has more at stake. Obviously, this would be true for people who are themselves religious believers. But I think it is also true for atheists and agnostics who come from a Christian cultural background. Long, long ago I wrote a post comparing HP Lovecraft and MR James. As I put it then, there is something about MR James's horror, which assumes a kind of default Christian backdrop to events, that gives it a much greater sense of immediacy and resonance than HP Lovecraft's entirely invented mythos:
James's universe is one where things make a kind of sense, even though he was expert in keeping things hidden. The ghosts, spirits, demons who his protagonists encounter are products of Christianity; it's a vicious, vengeful, Old Testament Christianity, where sins are punished rather than forgiven, and it's a Christianity which comes more from the Apocrypha (The Testament of Solomon, Knights Templar, medieval Jewish magic) than from the Bible, but it's still a universe people from the Western world are familiar with. It's in many ways a quid pro quo universe - you get what's coming to you - but more importantly it's one that's horribly familiar, especially if you have had a church upbringing. Words like Baphomet, Satan, King Solomon, hell, the afterlife, altar, pew, prayer book, etc., have meanings to us which extend beyond the immediate story or what the writer can conjur up, and reach into our shared Judeo-Christian cultural past. This gives them a sense of weight, a sense of meaning, that made-up words like Hastur do not.
You don't have to be religious to appreciate that certain shared myths, stories and artefacts can take on a sense or feeling of the numinous, despite your own agnosticism: they get it not from the fact that they're true, or genuinely 'spiritual', but from something deeper - they've been around a long time, thousands of years in some cases, and when something is around a long time, it tends to grow roots. The Testament of Solomon is spellbinding because these are stories which have their roots in extreme antiquity, and something that old can't help but feel significant.
The point here is that one doesn't have to be a Christian to feel the viscerality of the notion that an orc or gnoll is a son of Cain rather than an evil humanoid - or indeed that a pit fiend is satanic rather than 'chaotic' or whatever else. One gets the concept of fighting a chaos demon; but one feels, in fighting a servant of Satan, that something weightier is going on. I think this would likely be truer across the piece, in a campaign setting which integrated Christian mythology (let's call it that for the sake of argument while sticking a pin in the question of metaphysics) in a more direct way.