Priesthoods are also restricted, usually by decree of the god himself. Below are examples of many typical sorts of restrictions: Note that most priesthoods will only have a few of these restrictions, and each faith may employ different ones.
Gods make these requirements of their priesthoods for four principal reasons: Commemoration, Function, Philosophy, and Sacrifice.
Commemoration means that the action is a reminder of some important event from the history of the faith. For example, the cross and crucifix are symbols of the Crucifixion.
Function means that if the requirement is not met, the priest cannot for some reason function as a priest. For instance, if, in a specific campaign world, clerical magic will not work if the caster has consumed alcohol, then the priesthood will have a requirement that its priests not drink.
Philosophy means that the choice is bound up with other elements of the faith. If certain creatures are held to be unholy, unclean, or otherwise taboo, for instance, there will be many secondary requirements derived from that thought. A priest wouldn't be able to wear armor made from its hide or eat its meat. To be buried in or with its skin might even result in involuntary separation from the faith!
Sacrifice means that the god requires this behavior to test the mettle of his followers. Those who can't make the sacrifice are obviously not cut out to be his priests.
At his discretion, the DM can make any of these requirements apply to the worshippers of the god in addition to the priesthood. Especially appropriate are restrictions involving Contamination (see below).
Armor[]
Priests may be limited in the types of armor they wear. Some sample limitations:
May not wear non-metal armor
May not wear metal armor
May not wear magical armor
May not wear any armor
May not use shields
May not use certain types of shields
May only wear armor made by priests of the same faith
Any of these restrictions could be made for any of the four reasons given above. If metal armor disrupts clerical magic, then it could not be worn by priests. If the religion's philosophy forbids harm to animals, then leather armor may be forbidden. And so on.
Armor restrictions don't just reflect the god's attributes or prohibitions; they help define the combat roles of priests in the campaign. A priest who can wear full metal armor is more likely to be a combat force in the campaign than one who can't.
You can be more sure that a player who picks a priest-type which is limited to less efficient armor is a player who wants to role-play a priest, rather than one who merely wants to play a fighting machine who is efficient in both combat and magic.
Celibacy[]
Though in modern times the terms Celibacy and Chastity have become confused, here we're only using the older meaning of the word Celibacy: The state of being unmarried. Priests who are required to be celibate must remain unmarried. A DM must decide individually for each celibate priesthood if its priests must also remain chaste (see below).
A priesthood could require celibacy as a sacrifice to the god, because it was philosophically opposed to the state of marriage, or for many other reasons. The fighting priesthood of a war-god might require its priests to stay celibate so that they won't be distracted by thoughts of home and family while engaged in warfare.
Chastity[]
Chastity involves not engaging in sexual relations. A priest could be celibate but not chaste; one could even be chaste but not celibate, though that would be pretty strange.
Priesthoods require chastity as a sacrifice to the god, or when its priests are supposed in some way to be spouses of the god (either in a symbolic or genuine sense).
In some faiths, chastity is required of its priests except for during specific events or times of the year. For instance, priests of an agricultural deity might be required to remain chaste except during the planting season, when chastity is revoked in order to magically "encourage" the fertility of the fields.
Clothing[]
Priests are often required to wear distinctive costumes proclaiming their status. They may only have to do so during the performance of their official functions, or might have to wear their priestly vestments during all waking hours.
Such items don't have to be full costumes. A priest might be only required to wear the specific holy symbol of his faith; otherwise, he could wear what he wished.
In some faiths, priests cannot wear certain types of clothing. Historically, some priest-kings of earth-goddess were forbidden to wear clothes with knots in them; if they wore textile garments, they had to have ragged, unknotted hems.
Priests might also be required to conceal certain parts of their bodies by clothing. Beyond restrictions imposed by society for modesty's sake, priests might have to conceal other parts not considered immodest by the general population.
Contamination[]
Many faiths regard certain items or substances as unholy, unclean, or taboo. Its priests are not permitted to handle such things. If they come in contact with them accidentally, they must undergo holy rituals of purifications to cleanse themselves of the taint.
Some sample items or substances include:
Animals (specific animals or whole classes of animals)
Blood
Gems or Jewels (specific types)
Iron
Plants (specific plants or whole classes of plants)
Water (from specific bodies of water)