Now, we get to a topic which adds a lot of color to the priesthood and the campaign world.
All priesthoods have certain duties to perform, duties to the god and duties to the community or civilization. The DM needs to decide what each priesthood's duties are, and will often be able to use those duties to tie the priest character in to specific adventures and role-playing situations.
Following are some sample ideas for priestly duties. The DM can use as few or as many of these as he wants when designing a new priesthood; he can also invent new ones to suit his campaign.
Devotions[]
These are ordinary prayers and rituals which the priest conducts on a regular basis. They might include the saying of prayers at specific times of the day, upon observation of specific incidents or natural phenomena, and so forth.
The DM can decide what these are and when they're undertaken, but the campaign shouldn't devote a lot of time to them; it's annoying and disconcerting to interrupt the adventure every so often so that the priests can pull out their holy symbols, kneel to the sun, and lead the faithful in prayer. Just knowing that they do this sort of thing on a regular basis is enough for most players.
Guidance[]
In most civilizations, priests are supposed to act as counselors to everyone in need of advice. (This is one reason that Wisdom is a prime requisite.)
Therefore, the priest character should not be surprised when he finds NPCs seeking him out and consulting him on troubling moral and ethical problems. These are good role-playing opportunities; they are often leads to specific adventures; and, with especially knotty problems, they can be difficult puzzles for the players to solve, all without using their swords and maces.
Often, priests are posted to specific noble leaders in order to be their personal advisors. Naturally, this is only done when those noble leaders desire it, are willing to allow it, or (because of the priesthood's political strength) cannot afford to refuse it. This is a way to establish some sort of bond between new player-characters: The priest player-character could be assigned as advisor to the noble player-character.
Marriage[]
In many human cultures, only the priest can perform marriages, so the DM may wish for this to be a duty of priests in his campaign.
The DM will need to decide for his campaign whether or not marriages require the ministrations of priests, and might make a different choice for each sentient races. If humans require priests for marriage, do dwarves? (Perhaps they use advocates and notaries instead.) Do elves? (Perhaps their marriages are officiated by bards, who write songs commemorating the event as a sort of "marriage certificate.") Do halflings? (Perhaps they prefer ceremonies presided over by sheriffs or judges.)
Also, the DM can always decide that priests of certain faiths cannot perform marriages. What, for example, does the god of weaponmaking have to do with marriage? On the other hand, it's equally appropriate for priests of any god to be able to perform marriages. This is especially true if, in the campaign, marriage constitutes permission or recognition from "higher forces," and any god, including those with attributes unrelated to marriage, may bless a marriage.
Missions[]
The duties of priesthood often involve going on missions important to the welfare of the god or the priesthood in general.
One of the most common missions involves going somewhere and trying to convert the local population to worship of the priest's god. Usually, this involves religious education and what amount to social services; sometimes, it involves conquering that culture and ruthlessly suppressing all signs of its old religion.
Other, more exciting missions can involve recovery of artifacts, transportation of persons or goods (such as temple money) through dangerous territory, interpretation of phenomena in distant places, and holy war with the followers of another god.
It's important for the DM to remember that the god himself isn't the only one who sends priests out on missions. For most missions, it's the upper ranks of the priesthood who do the assigning, and priests are supposed to follow the orders of their superiors. So it's possible for any priesthood to have a "bad egg," a priest who issues orders which are contradictory to the tenets of the faith or designed to promote only his personal power. This should, however, be a very rare campaign event, unless the campaign revolves around uncovering and purging a corrupt element of the sect.
Omen-Reading[]
In some campaigns, priests will be charged with the duty of reading omens for the future.
If a campaign's priests have that duty, the DM has to decide how they do it, what it is they're actually doing, and who they're doing it for.
How They Do It[]
Omen-Reading always requires some sort of ritual, usually a public one.
The priests may sacrifice animals and examine their entrails for clues to the future. They may read tea-leaves. They may inhale dangerous fumes and prophesy while under their influence. They may listen to whispering in the trees, babbling of brooks, or the singing of birds and interpret that noise. They may enter meditative states and wait for inspiration from the gods. Each cult could do it a different way, and the DM can choose the method which he feels will add the most appropriate color to the cult in question.
What They're Doing[]
Then, the DM has to figure out what it is the priests are actually doing when prophesying. Here are some choices:
They're Receiving the Word of Their God: The priests are actually receiving some inspiration from their deity. Naturally, such omens are usually clouded in imprecise terminology and symbolism, so that it's easy for the recipient of an omen to misinterpret the results.
They're Following Ritual Interpretation: The priests have a set of techniques of interpretation which they follow rigidly. These techniques may or may not have any basis in campaign reality; they might have been granted by the god, or created through ignorance.
They're Analyzing Based On Their Knowledge: The priests aren't actually prophesying at all, but supplying answers based on their understanding of the situation and of the ways of the world. If they are then pretending that their answers come from a higher source, they are obviously being dishonest; only a corrupt branch of a priesthood will do this. However, it's possible for such a method to be very accurate, especially if it concerns itself mostly with questions of warfare and human nature.
They're Telling the Audience What It Wants to Hear: The priests are acting primarily as cheerleaders and telling the people precisely what they want to hear: That they'll win the war, they're always right, they've done no wrong, love conquers all. Again, priests acting in this manner are being dishonest to their flocks, but it will often be difficult to convince the flocks of that.
They're Working for Gain: Some very corrupt sects or branches of sects manipulate their answers to gain in power or money. This usually takes one of two methods.
In the first, the priests supply answers which favor their purposes. If representatives of one city ask, "When we attack our enemies, will we prevail?" the priests then decide whom they want to win that war. If they want the attackers to win, they answer "Yes." Then, the attackers will be encouraged by the reply, and the defenders discouraged, which weighs the war in the attackers' favor. If they want the defenders to win, they answer "No," with precisely the opposite effect; the discouraged attackers might not even launch the attack.
In the second, the priests accept bribes in order to put their god's stamp of approval on the activities of certain men. For instance, a king might secretly pay the priests a lavish amount, then publicly approach the oracle and ask, "Shall I not execute the traitor so-and-so now without benefit of trial?" or "Should I marry so-and-so against her wishes?" or any other such question. The answer he receives, of course, will be the one he paid to get, and because the god has "made his wishes known," the citizens will probably not dispute the choice.
In both these approaches, the god may eventually notice that one branch of his priesthood is corrupt, and set about correcting matters, either through his own intervention or by alerting other branches of the priesthood. On the other hand, a particularly disinterested god might never notice.
But Are They Right?[]
As part of deciding what they're doing, the DM has to decide how often they're right.
The priests will often be right if they're receiving the word of their god; this word may be misinterpreted, but it's always correct.
They could have any sort of accuracy the DM decides if they're following some pattern of ritual interpretation; perhaps the ritual is effective, perhaps it is not.
If they're analyzing the situation based on their current knowledge, then their accuracy depends mostly in their interpretive abilities; a priesthood might have enough knowledge of the world and human nature to be able to supply consistently-correct answers to supplicants.
If they're telling the audience what it wants to hear, they could be very accurate for a time, especially if they're prophesying for a warlike state which is on the rise and mostly asking about upcoming victories. Eventually, however, the tides of fate will turn and the prophecies will become unreliable, which will disillusion the populace.
If they're working for gain, they could be very successful for quite a while. Eventually, though, the scandal will break, and the population will learn the truth... which could be very bad for those greedy prophets.
Who It's For[]
Finally, the DM has to decide who is able to receive these prophecies. Here are some typical choices:
Anyone: Anyone who asks a question will receive some sort of reply.
Anyone With the Means: Some temples require a sacrifice of animals or wealth in order for the supplicant to receive a prophecy. (This isn't necessarily a sign of corruption; it's often just a means of ensuring the temple's upkeep and the faith's continuing secular, or worldly, power.)
Nobles Only: In this arrangement, only members of noble houses can ask questions of the oracle.
The DM can also make more peculiar choices for specific oracles. An oracle might only be for slaves, for adventurers, for people who have at least once travelled to a specific holy site, for people of specific alignments, for members of one race, etc.
Vigilance[]
Finally, priests have to be vigilant against powers or elements which threaten their faith or their followers.
These powers and elements don't usually take the direct approach, such as attacks by armies or monsters. The priest needs to be vigilant against more subtle intrusions, including:
The Agent Provocateur[]
This is someone who falsely joins the priesthood, spends a long time becoming a trusted member or even a leader of it, and then persuades members of the faith to perform actions which will get the faith into trouble. For example, in times when the priesthood is in conflict with another faith, the Agent Provocateur might encourage outright war with that faith. When a conflict with the local rulers could be sorted out by calm diplomacy, the Agent Provocateur will instead recommend or issue ultimatums and demands. And, naturally, the Agent will keep his true masters apprised of the priesthood's secret movements and activities at all times.
Corruption in Specific Orders[]
Sometimes priests go bad and use the priesthood for their own gain. In addition to methods mentioned above, they may also secretly defy requirements of their priesthood, steal temple funds, use their duties of guidance to influence others to profit these priests, etc. No such corruption goes unnoticed forever, but the unwillingness of people to believe that they've put their faith in crooks and thieves can ensure that this corruption can go on for a long, long time.
Libels[]
At times, members of rival priesthoods will persuade their followers that other priesthoods perform acts which are profane and evil. In a culture where religious prejudice is a strong factor, this is often easy to do. For instance, it might prove simple to convince one's flock that the priests of a more despised faith are kidnapping young women (only women of the libeller's faith, of course) to serve as unwilling temple concubines, then killing them. It's especially easy to do when the libeller secretly arranges for the kidnapping and murder of several young women in his own faith. When this sort of thing goes on, it's vital that the priests of the accused faith prove the truth. They can't do it just by giving local authorities a tour of their temple to show there are no unwilling concubines here—and even if they do, a particularly clever enemy will have concealed the body of one of the kidnapped girls there for the authorities to find! It requires capture of the killers and demonstration that they were serving someone else... all of which is a good basis for a priest-oriented adventure.
Obviously, it's the duty of faithful priests to combat all these situations these when they're noticed. However, it's a characteristic of the greatest priest-heroes to notice these trends well ahead of the time that they become critical, and to deal with them early in their development, before they can result in serious harm to the priesthood.
For the DM, this is a good way to give the PC priest an opportunity for rapid advancement in his priesthood: If he's the first to notice such a trend and is able to spearhead the movement to correct it, he will be well-regarded by his peers. This is also a good way to establish that an NPC priest is a hero of his faith, if he, in the past, has thwarted such situations.