Murdundia and Monotheism

I think the first campaign setting I ever created for a role-playing game was the World of Murdundia for Dungeons & Dragons. It was a fantasy world loosely inspired by medieval Europe (as is typical), and it was dominated by a monotheistic religion strongly based on Christianity (which was atypical for fantasy games that were not Arthurian). It wasn't that I had any aversion to polytheistic settings—I was a big fan of the World of Greyhawk—but the desire of one of my players to play a paladin, coupled with the strong presence of infernal monsters in the Monster Manual, led me to envision a world closer to the historical Middle Ages than Middle Earth, the Young Kingdoms, or the Hyborian Age. (The fact that I once had the sun blotted out by a swarm of byakhee is beside the point.) The monotheistic religion was called Veritas (Latin for "Truth") or, less-believably (and less pronounceable), Veritasism. Its antithesis was diabolism (the worship of devils) and demonolatry (the worship of demons, primarily Orcus), both of which were at odds with one another. FrDave's Blood of Prokopius has caused me to seriously consider resurrecting this campaign setting. Polytheistic settings are great, especially when the pantheons are fascinating, but there is so much to be mined from all the history, literature, art, architecture, folklore, and cultural traditions of monotheistic medieval Europe that it would be a pity not to use it for at least one setting where paladins, clerics, demons, and devils are important. Murdundia might have to be one of my OSR projects. A better name for the religion might be in order, though.