I wanted to discuss the religions of the Nightwick Abbey campaign... so here we are. From a setting perspective, I wanted religions which would both allow me to engage with some of the historicity of the Middle Ages, and feel like Clark Ashton Smith's Averoigne cycle. This is really the first time I've decided to buck D&D's pseudo-traditional polytheistic assumption for something more closely resembling Medieval religion (as viewed through a lens of Gothic horror).
Another design goal was to have one religion for each alignment. For my campaign I'm using the traditional Law/Neutral/Chaos split. The religions have also been designed, largely, to incorporate the Druid in a way that takes out some of their hippy-dippy-high school Wicca-nature-happy-fun-time bullshit. As such I, obviously, need three religions. They are detailed below.
The Great Church represents the forces of Law. It is essentially Christianity with all references to Jesus removed. They're monotheistic, headed (loosely) by a pope, have holy orders of various sorts, and like to evangelize. The Church ordains women as well as men, making it at least a tad different from the Latin Church of the Middle Ages. Their symbol is either going to be a cross, or a vaguely cross-shaped star burst used to indicate the sun. Nightwick Abbey was dedicated to God before it fell into darkness. All non-evil Clerics are members of the Church. This is not to say that the Church is solely a good institution. Pastors may be just as easily tempted as their flock, and the servants of the dark ones have infiltrated deeply into the Church's infrastructure.
The Old Gods were the only gods worshiped in the campaign area before the Church arrived. There are many, especially outside of the towns, who still conduct the ancient rites to pay homage to the Old Gods. The Old Gods are not worshiped, so much as placated. They represent natural forces and demand various types of sacrifice, including human sacrifice, in order to spare the realms of humankind. They are, however, capricious and have been known to aid heroes as well as hinder them. The mysterious Druids are their priests and priestesses. It is unlikely that I will ever detail specific Old Gods for anything other than a quick description. They are always worshiped together, and the peasants who still take grain to their altars simply call them "the gods."
There are many demons. They writhe and wait in ink black pools to lure humans to their doom. They are worshiped by mad and desperate people who believe that doing so will win them power or glory or fame or sex or other, darker desires. Demons seek to unmake the world a pull it back into darkness so that they no longer feel the eternal suffering of their existence. The "god" referred to in my previous post is actually a demon. For the moment I'm assuming it is Orcus, but that may change as the campaign develops. Anti-Clerics obviously are the worshipers of demons; however, it can often be difficult to tell one from the other. Unlike good Clerics, Anti-Clerics can both heal and harm. As such, they can masquerade as the righteous and lead many on the road to damnation.
The campaign area is kinda-sorta modeled on the Balkans during the Northern Crusades. The Great Church has already made quite a few converts, including chieftains and kings, but the fall of Nightwick Abbey to diabolic forces has shaken them. Many wonder if the worshipers of the Old Gods would have been better off if the Church had never arrived, and argue that the Old Gods are an expression of the same divine force.
In the cities, most people worship God. A few farmers who come in to trade worship the Old Gods, but they are wary of making this fact known. Even those who have been converted are usually very confused as to just what the difference between an Old God and a Saint is. In the country side the Old Gods still hold sway, though the Church is attempting to make headway once again. Demonists and Satanists hide everywhere, waiting to devour humankind and lead it to damnation.
Edit: Though I would have had a pseudo-Christian main religion anyway, I should give credit where credit is due and note that I stole the name of the Great Church from Scott, at Huge Ruined Pile, who in turn stole it from B2. Just so we're clear.