A priest is a blessed mortal who serves as a link between his people and the divine. Priests are literate and, as the scribal administrators of civilization and the guardians of cultural knowledge, they have general education in a variety of useful topics. The primary function of a priest, though, is performing the rituals that maintain divine favor and access the knowledge and insight of the spiritual world. Powerful priests can even perform great miracles. Most priests are not very proficient in combat, and need to be protected in a melee.
Game Mechanics
Priests have two game-mechanical functions, theurgy and divination.
Theurgy: Theurgy is inducing the gods to perform particular miracles. This is in a sense an economic transaction: the priest buys miracles with sacrifices to the god. The value of the sacrifice depends on what it is, and how well the priest offers it up to the god. Gods may also grant miracles for lesser, or reduced, payments if their interests are sufficiently aligned with those of the priest. Different gods have different proclivities for types of miracles, different favorabilities toward particular religious traditions and types of sacrifices, and varying popularity. More widely worshipped gods are more powerful (because they receive more sacrifices and are attended by better priests), but also more likely to have extensive existing commitments. Powerful deities, near the top of the divine hierarchy, tend to be choosy about priests and sacrifices; ones farther down on the hierarchy are less picky and will usually take what they can get. Deities can be offended by conduct they oppose, alienating them from otherwise-acceptable clergy and offerings, or conversely may look with favor on a lowly but pure-hearted cleric who is asking for a miracle that would further the divine interests.
Serendipity involving the Sacrifice skill roll may involve receiving gifts or boons not specifically asked for, besides a higher than normal yield of divine favor from the sacrifice.
It is allowable for a priest of one god to sacrifice to, or invoke, that god's hierarchical superiors or inferiors, but invoking a "sibling" deity may incur a variable amount of disfavor, depending on how jealous the deity in question is and their opinion of the cross-invoked deity. The cross-invoked deity, too, may be flattered by attention by an outside priest making a sacrifice, or offended by a comparative stranger asking for a miracle, depending on his or her personality and what is offered or asked.
The Hallowed Immortals of the Serene Goddess have more or less fixed relationships to each other. The Manifold Spirits of the Wild God, however, have more tumultuous relations, which tend to change on the full moon, when they banquet together and grow their friendships and enmities. Mortals do not have a fly on the wall of the banquet hall, so without preparatory divination, the consequences can be unpredictable. Serene and Wild deities always have 0% favor toward deities in the other faction.
Deities each have a percentage favorability toward each other. They are always 100% favorable to their own hierarchical inferiors and the deity of which they are an inferior (if applicable.) Every full moon in-game, the GM adjusts the favorability ratings.