Ethos and Requirements of the Faith (CPrH)

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Each faith requires certain codes of behavior, beliefs, and even abilities of its followers and of its priests. When creating a faith, you have to decide what those requirements are and how they're to be implemented in your campaign.
==Goals and Purposes==
First, what are the goals and purposes of the god (or force or philosophy), and therefore of the faith?
Often, that's self-evident, because it's usually tied to the attribute of the god, or the explanation of the force or philosophy.
For example, if the faith's god is a God of Love, then the goals and purposes of the faith will probably include:
Promotion of Love, which might include the helping along of lovers, opposition to those who interfere in the development of romantic relationships, punishment of those who defy the god and refuse to love, etc. This could also include "social services" to the community, through the god's temples: Counseling to young lovers, for instance.
Promotion of the God, which includes the building of temples or churches, carrying the word of the god to those who have not heard it, and keeping the stories of the god ever-present in the ears of the population. 
Opposition of Enemies, which means subtle or direct opposition to enemy gods and their followers; obviously, a god of Love is likely to be opposed to a god of Hatred, Misanthropy, Misogyny, etc.
Those are some basic goals, but you obviously aren't limited to goals which are that direct and simple.
For instance, a god may represent only a small part of his attribute. A God of War does not have to be just a god of all types and elements of warfare. He could be the God of the Chaos of War, the God of Intelligent Warfare, the God of Naval Warfare (in which he might share traits with a god of Oceans), the God of the Sword (in which case he might share traits with a god of Metalworking), and so forth. If you choose, you can always make a god's personal attribute more specialized, and can choose more specialized goals for the faith based on that choice.
In addition, a god isn't defined just by his attribute. In the campaign history, he also has a personal history, likes and dislikes, relationships with other gods, and ambitions, all of which can provide more goals for the faith.
As one example, Zeus, the king of the gods of Greek mythology, had many attributes and associations. He was the king of the gods, a sky-god, a god whose emblem and weapon was the lightning bolt, a god of Wisdom (he had swallowed and absorbed the wisdom-goddess Metis, mother of Athena), a god of oracles (though he was eventually supplanted by Apollo in this role, he had a major oracle at the city of Dodona), a protector of fugitives, a lover of many goddesses and women...  In short, he had many characteristics and attributes, and in an AD&D® game campaign his priesthood would embody and promote most or all of them. 
==Alignment==
A faith will often demand that its priests belong to a specific alignment or a limited range of alignments. The DM determines this, usually basing the choice on the attributes and character of the god, force or philosophy in question.
But don't be too restrictive in this regard. Even if, personally, you're opposed to War, the God of War and his followers don't have to be Chaotic/Evil.
==Alignment Guidelines for the Priesthood==
Here are some general guidelines to go by:
The first, and most important, note is this: The gods usually do not insist that their priests be of identical alignment to the god. The alignment may not be so dissimilar that the priest cannot serve the god, but it does not have to be identical. 
If the faith does not promote any sort of harm to living beings, or promotes benefits to living beings, then it probably excludes Evil alignments among the priesthood. If the faith allows for harm to living beings but does not promote wanton cruelty, then it can include Good, Neutral, and Evil priests. If the faith does promote cruelty, then it probably excludes Good alignments among its priests.
If the faith demands ordered thinking, strict obedience to laws, and unquestioning acceptance of policy, then it leans toward Lawful behavior. (Now, every faith requires rituals and devotions of its priests, but this isn't the same thing, and doesn't require any alignment choice.) If the faith demands more free-willed and spontaneous behavior, defiance of social conventions or restrictions, and so forth, then it leans toward Chaotic behavior. If it promotes neither approach strongly, then it can probably include Lawful, Neutral, and Chaotic choices.
Here are some sample choices:
God of Love: This type of faith usually promotes no harm to living beings, and promotes the benefits of love; it often promotes free-willed and spontaneous behavior, but not strongly. Therefore, its priests will probably be required to be Good, and may be of Lawful, Neutral, or Chaotic alignments, though the tendency is toward Chaotic. However, if the god is a god of passionate affairs and selfish love, regardless of who gets hurt, and if he promotes revenge on romantic competitors and ex-lovers, then the faith could well include Evil and Neutral priests, thus giving its priesthood the full range of alignment choices.
God of War: This faith generally allows for harm to living beings without promoting wanton cruelty; and warfare runs the gamut from carefully-reasoned strategy to wild, bloodthirsty battlefield chaos. Therefore, the faith probably places no restriction on the alignment of its priests. However, a specific god can be devoted to one aspect of war. For example, a god who promotes the bloody destruction of all enemies, including sacrifice of prisoners and innocents, will probably have an Evil priesthood. A god who is the god of military planning will probably have a Lawful, or Lawful and Neutral, priesthood.
==Alignments of the Worshippers==
Most faiths define various acts and types of behavior as evil and demand that their followers not perform those acts. Some few make those same definitions and demand that their followers do perform them. Almost no faiths demand that their followers belong specifically to Lawful or Chaotic alignments.
Therefore, most faiths require of their (non-priest) worshippers that they be anything but Evil. A very few faiths require instead that their worshippers be any sort of Evil (Lawful/Evil, Neutral/Evil, or Chaotic/Evil).
==Ability Scores==
Most priesthoods demand some sort of minimum ability scores of their priests.
The prime characteristic of priests is Wisdom. To be a priest of any sort whatsoever, the character must have at least a wisdom of 9. Some priesthoods will require that the wisdom score be higher, though usually not higher than 13. 
Generally, if the priest's Wisdom is 16 or better, he gets a +10% bonus to his earned experience.
Many priesthoods will require a second prime requisite. For example, priests of a god of War may have to have a certain Strength score, while priests of a god of Magic might have to have a certain Intelligence score. Usually, this second prime requisite must have a score of at least 12; up to 15 is not an excessive requirement.
In such cases, the DM may decide that the character, if he has either the Wisdom score or the other Prime Requisite at a score of 16, gets a +5% bonus to earned experience, but if he has both, he gets a +10% bonus.
In the next chapter, you'll find numerous examples of such priesthoods and recommended minimum ability scores for them.
==Races Allowed==
The DM may wish to limit certain priesthoods to certain races.
This is almost always a choice based on the history of his own campaign world. There is no game-related reason why most races can't have priests for any god, force or philosophy; but there are often campaign-related reasons why a certain race can't belong to a certain priesthood.
For example, if the halflings of a specific campaign world are pacifists, they'll be excluded from priesthood in the faith of the war-god. If dwarves are pragmatic, unromantic sorts who arrange all their marriages and don't conduct affairs of the heart, they'll be excluded from priesthood in the faith of the love-god.
In the next chapter, we provide numerous examples of priesthoods of specific mythoi. With each, there's a recommendation for allowed races. These recommendations are based on the most common and popular conceptions of these races, and the DM is free to change them for his specific campaign. 
Players beware: When the Complete Priest's Handbook says one thing about allowed races, and the DM says another, the DM is always right.
==Experience and Spell Progression==
It would be possible to come up with an Experience Levels Chart and Spell Progression Chart for every priesthood of specific mythoi, but it would also be crazy; there's no reason to have the extra complication in your campaign.
All new priesthoods introduced in the next chapter use the Cleric experience progression and the basic Priest spell progression, both from page 33 of the Player's Handbook. If your DM, when creating a new priesthood, decides that it is observably less powerful than the Cleric or the priesthoods written here, he can choose to use the Druid experience progression, which allows for faster acquisition of experience levels.
==Gender Requirements==
In the worlds of the AD&D® game, most priesthoods should allow both priests and priestesses. However, in fantasy worlds and the real world, some faiths have required that all their priesthood be of just one sex. If there is such a restriction on a given priesthood, the DM will make note of it and tell the players. 
==Nonweapon and Weapon Proficiencies==
Various priesthoods will require priests to know certain skills (nonweapon proficiencies) and be able to wield certain weapons (weapon proficiencies). For example, a priest of the god of Agriculture must know the Agriculture proficiency, while a priest of the god of Fire must know Fire-Building.
Every priesthood should require one nonweapon proficiency of its priests and priestesses; it's a sign of their devotion. It's permissible, but not recommended, for them to require more than one.
A priesthood may require one or more weapon proficiencies of its priests and priestesses, but we don't recommend this for most priesthoods. Many faiths allow their priests so few weapon choices anyway that it's pointless to require they be taken. An exception is when a god is noted for wielding a specific weapon; for instance, it's quite reasonable to require Weapon Proficiency in War-Hammer for priests of the god Thor, whose principal weapon and symbol was the hammer.