Now, I realize that, especially when it comes to Clerics, Hammer Horror films, especially those that depict Dr. Abraham Van Helsing (usually played by Peter Cushing) as a vampire hunter, usually opposed by Christopher Lee's Dracula (I actually chipped away at the list of classic influences on D&D I, as a "grognardling," haven't experienced by watching the original Horror of Dracula yesterday) are a major, if not the primary influence, and I don't think that they are without value even in a setting where Clerics and Paladins are members of religious orders instead of professors independently seeking out and destroying the undead. For example, Van Helsing's personal mission sounds an awful lot like, "seeking out and destruction of evil heretics [substitute "vampires" here] and their lands and also of those who rebel against the faith of the holy church." The wording comes from the mission of the Militia of the Faith of Jesus Christ, formed mainly to combat Cathars in southern France. Van Helsing even goes so far as to call vampirism a "cult" in Horror of Dracula.
Van Helsing, though, is obviously not the only inspiration for D&D's Clerics, as Clerics in D&D are not (at least not universally and primarily) professors, but are clergy in some religion. I've read multiple places that Odo of Bayeux (shown wielding a mace in the Bayeux Tapestry) and Bishop Turpin, a Paladin companion of Roland, are also inspirations for Clerics.
It makes sense, then, to look at actual religious orders to get inspiration for beefing up the background, organizational, world-building side of Clerics and Paladins, especially since the trend it my world-building currently concentrates on tying PCs to the setting through various class-based organizations. In practice, this has meant a lot of looking through Wikipedia, trying to figure out the difference between Clerics Regular and Canons Regular and trying to figure out what kinds of orders might produce adventurers in a fantasy setting. I don't really have enough to show anything of value for Clerics yet, but I chose to look concentrate on military orders (seeing as there are fewer of them than non-military Catholic orders) and have a few things that might be profitable for gaming even in this early stage of my research.
For one thing, the purposes of military orders varied widely, and most seem to have more than one purpose:
- Providing care in a hospital (which might specialize in a certain awful disease, like leprosy) and protecting the hospital with force
- Protecting pilgrims to certain lands or certain holy sites
- Reclaiming lost territory from infidels
- Ransoming captives (this seems to have meant soliciting from donors)
- Ensuring the proper burial of fallen Crusaders
- Defending frontier areas from infidels
- Inquisition and invading areas where heretics ruled
- Keeping the peace in a certain area
- Improving ties between the Church and the nobility and rulers of an area
- Defending the rights and freedoms of the Church