In 1e Gygax says that the class resembles historical clerics, but that they follow a god or gods. I seriously doubt that he had a sudden reversal from Basic to 1e.
Gygax didn't write Moldvay Basic - Tom Moldvay did, and after AD&D was published.
The reference to following "a god" is from Moldvay Basic. And, as I've already posted, it is addressed to player and referees - it tells us (via the indefinite article) that the rule system contemplates the existence of the divine, but not any particular divinity. It doesn't tell us whether or not the gameworld is, by default, polytheistic or monotheistic.
(It does take for granted that their are Chaotic clerics, the analogue of what used to be called "anti-clerics", but it is easy to conceive of the "dark gods" they worship as not really
gods at all in any strict sense. Prototype warlocks, if you like - with obvious allusions to real-world, mediaeval understandings of witchcraft mixed with various cultists of dark gods from pulp stories.)
It doesn't all mean that they were going for monotheism, or that monotheism was the default. Clearly from the other references, polytheism was what they were going for.
No one is asserting that monotheism is the default. I'm just saying that it is clearly on the table, just as much as polytheism is. The classes support it, the game rules don't get in the way of it. Whereas polytheism is
hard if you don't want a cleric of Poseidon and a cleric of Hermes to look identical, because the rules of the classic game provide no resources for differentiating them.
No one is disagreeing that you can play D&D in a monotheistic world. Even if some editions might be easier than others, I'm still going to argue that none of them ever really supported it. At best, they didn't really mandate either way, but, outside of "Well, do it yourself", there is very, very little support in any edition for monotheistic settings.
What support is missing? We have warrior-saint/crusader-type PCs in two different mechanical variants (paladin and cleric). That's all that's required!
(If you mean that all the settings are polytheistic, that's true. But if I want to run (say) Keep on the Borderlands, which came with my Moldvay boxed set, I don't need a setting. And the Keep works perfectly well in a faux-mediaeval monotheistic setting.)
By the way, I looked through the AD&D PHB for discussions of clerics and gods. Here's what I found:
This class of character bears a certain resemblance to religious orders of knighthood of medieval times. . . . The cleric can be of any alignment save (true) neutral . . ., depending upon that of the deity the cleric serves. All clerics have certain holy symbols which aid them and give power to their spells. All are likewise forbidden to use edged and/or pointed weapons which shed blood. All clerics have their own spells, bestowed upon them by their deity for correct and diligent prayers and deeds. . . .
When a cleric achieves 8th level (Patriarch or Matriarch) he or she automatically attracts followers if the cleric establishes a place of worship - a building of not less than 2,000 square feet in floor area with an altar, shrine, chapel, etc. These followers are fanatically loyal and serve without pay so long as the cleric does not change deities and/or alignment. . . .
Upon reaching 9th level (High Priest or High Priestess), the cleric has the option of constructing a religious stronghold. This fortified place must contain a large temple, cathedral, or church of not less than 2500 square feet on the ground floor. It can be a castle, a monastery, an abbey or the like. It must be dedicated to the cleric's deity (or deities). construction will be only one-half the usual for such a place because of religious help. (p 20)
Clerical spells . . . are bestowed by the gods, so that the cleric need but pray for a few hours and the desired verbal and somatic spell components will be placed properly in his or her mind. First, second, third, and even fourth level spells are granted to the cleric through meditation and devout prayer. This spell giving is accomplished by the lesser servants of the cleric's deity. Fifth, sixth, and seventh level spells can be given to the cleric ONLY by the cleric's deity directly, not through some intermediary source. Note that the cleric might well be judged by his or her deity at such time, as the clerk must supplicate the deity for the granting of these spells. While the deity may grant such spells full willingly, a deed, or sacrifice, atonement or abasement may be required. The deity might also ignore a specific spell request and give the cleric some other spell (or none at all). Your Dungeon Master will handle this considering a cleric's alignment and faithfulness to it and his or her deity. (p 40)
All material components required for the various spells are used by completion of the spell in question with the notable exceptions of standard religious items, i.e. religious symbols and prayer beads or similar devices.
The reversal of some spells might well place the cleric in a questionable position with respect to alignment. (p 43)
Oddly, it never actually
says that a cleric must serve one or more gods. It presupposes it. (As usual, Moldvay is a clearer rules text than AD&D.) And while there is talk of "the gods" and "a cleric's deity (or deities)", the actual conception of what a cleric's religion looks like is drawn entirely from a pulpy idealisation of mediaval Europe. This is a game that I think is trivial to run in a non-polytheistic gameworld.
