One of the assumptions made in gaming where any form of religion is involved is that monotheism is a later development. Animism and Ancestor worship develop into polytheism which then develops into monotheism. A great example of this is Avalon Hill's Advanced Civilization, where Monotheism is one of the most expensive Civilization Cards in the game. Thus, in context of a pre-historical fantasy age like that found in REH's Conan stories, it seems to make sense to default to a polytheistic world-building foundation.
This, of course, is not what is found in the Biblical narrative. Scripture suggests that the most ancient peoples were actually monotheists. Polytheism arrises later as civilizations grow and develop. Despite the apparent uniqueness of the Hebrews, this pattern even develops within the nation of Israel. The prophets are constantly warning the People of God to stay true to the ancient ways of monotheism instead of adopting the polytheism of the nations around them.
Recently, I came upon this video done by Inspiring Philosophy:
I realize it is 55 minutes long, but given the fact that most of us are isolated at home why not take the time?
It takes us through a bunch of evidence that suggests that the general shape of the Biblical Narrative is actually far closer to historic reality than what is assumed by the games we play. Again and again we find that primitive, nomadic cultures tend toward monotheism. Some even have strikingly similar stories and concepts found within the OT. Thus, the whole hypothesis that monotheism is a later development than polytheism is neither tenable nor realistic.
Thus, for the purposes of world-building a fantasy world inspired by Swords & Sorcery pulp stories about a Pangean/Hyperborean age it is actually quite realistic to use a monotheistic belief in a Creator god who spoke the world into being. This god might go by many names, but the fundamentals are pretty much the same across cultures.
If one wanted to add some polytheistic pantheons, these would be features of decadent city-states and urban areas rather than the more "primitive" areas outside the cities.
This creates an interesting inversion of my own classic campaign structure of Christian Civilization (Law) vs. the Demonic Wilderness (Chaos). Indeed, a campaign could be centered around retrieving objects and treasure hidden inside the City rather than the Dungeon and the classic fantasy and D&D trope of raiding a Temple could be a central activity.
In this context, I do find it quite ironic that the D&D ruleset itself mimics this historic development of religion. The farther back one goes, the easier it is to see the roots of a monotheistic influence on the game; however, as time goes on the game becomes more and more polytheistic not just in its assumptions, but in the rules themselves.
The real irony, though, is that the assumption made by both myself and Circas K that those later editions of D&D do a bad job of portraying clerics (whether you prefer monotheism or polytheism) may be less true than I ever thought.