At the DM's discretion, priesthoods can have special rights and powers, too. These rights usually depend on the amount of influence the priesthood wields in the campaign setting; a minor priesthood may not enjoy any of these benefits, while a dominant one could have all of them.
Church Trial[]
In some cultures, the secular (non-priestly) authorities cannot put priests on trial for any sort of crime. That doesn't mean a priest can perform a crime and laugh at the law, however: Most priestly orders police themselves, and will try (or punish without trial) transgressions.
Priestly orders don't ordinarily flaunt this power in order to defy secular authorities. When priests commit crimes, priesthoods usually punish them. Exceptions occur when the priest was acting at the priesthood's behest, particularly when the god requires actions which are against the law of the land. In these cases, the priest sometimes goes unpunished; sometimes he receives a "slap on the wrist" punishment to quiet the secular authorities.
The DM, not the player, gets to decide whether priests have the right to church trial.
Coronation[]
The most powerful priesthood in a given land will probably have the right to crown kings when they ascend the throne. The DM has to decide whether this is merely an honor bestowed on the priesthood, or whether it is a right which the priesthood can use to influence the throne.
If it's the former, then the chief priest is accorded the right to officiate at the coronation ceremony. However, if the priest refused to officiate, the crowning will take place anyway; the king can choose another priesthood for the honor, or he can be crowned without the priesthood's sanction.
If it's the latter, then the priesthood can deny someone the right to take the throne by refusing to crown him. This is a very powerful right, and through it the priesthood can exert considerable influence on the nation.
That's not to say that, by refusing to crown a certain candidate, the priesthood can stage a bloodless coup and choose the king.
Let's assume a priesthood does such a thing and refuses the chief candidate for the throne, then spreads the word they will support a certain other candidate.
The refused candidate may decide to mount a war against the priesthood. The new candidate, if crowned, may find that none of the other nobles in the nation support his kingship. All of this can result in a bloody civil war which could tear the nation apart and wreck the priesthood's power.
Therefore, when the priesthood decides to exercise this power, it does tend to make compromises, to negotiate secretly with the parties involved, to plan things carefully so that trouble is kept to a minimum. Only the most arrogant of priesthoods would try to take for itself the full right to choose the king... and such priesthoods are likely to lead their nation into war or chaos.
Other Confirmations[]
It could be that confirmations other than coronation are the special province of one priesthood. For example, in one nation, any priest might perform marriages for commoners, but only the priests of a specific god might perform marriages for nobles.
This would give that priesthood great power, because by collectively deciding or refusing to marry certain couples, this priesthood has the power to influence which families unite and which do not.
Again, abuse of this right could lead to harm, but careful application of it would allow the priesthood to affect the growth and development of the nation.
Tithes[]
Some priesthoods, the most powerful ones in a culture, are able to demand tithes of their followers. A tithe is an amount of money, often represented as a fraction of the money-earner's regular wage, which the follower is required to pay to the priesthood. Theoretically, it is used for upkeep of churches and temples, purchase of supplies and equipment for the priests, etc. Some priesthoods also use these moneys for influence with the government; a very few corrupt priests dip into it for personal gain.
In an AD&D® game campaign, only the religion of a monotheistic state, or the state religion, will be able legally to demand tithes of its followers. Such tithes will usually run from 5% to 15% of the character's income, with 10% being most common.
Priesthoods of other faiths will ask their followers to tithe a like amount. Naturally, not all their followers will tithe and so such religions bring in less tithed income than those who can demand it.
Player-character priests don't get to see that money; it is recorded by church accountants, stored in church treasures, and distributed or spent by superior priests.
Separation from the Faith[]
Some priesthoods can exert considerable influence on their followers by being able to separate them from the faith. If, for instance, all followers are promised a certain role in the afterlife, and separation from the faith would deny them that role (and replace it with something far more frightening or ghastly), then the followers of the faith are likely to obey the priesthood.
Naturally, most priesthoods visit this punishment only on those followers who most flagrantly defy the requirements of the faith. Just as naturally, the occasional corrupt priest would threaten followers with this punishment unless they are blindly, absolutely obedient to him.
However, this punishment doesn't work so well in a culture which worships many gods and which has a separate, independent priesthood for each god. If you threaten a follower of the war-god with separation, he might be just as happy to switch over to worship of the sea-god. This is a balancing factor which helps keep down abuse of this right in many cultures. However, monotheistic cultures (those which worship only one god) don't have this balancing factor; they have to rely on the honesty of the priesthood.
Rule[]
Some cultures are theocracies , which means they are ruled by their priestly classes. Naturally, in such a culture, one priesthood will be dominant; the culture may worship only one god, or may tolerate other worships but be mainly devoted to one specific god.
In a theocracy, the nation may be ruled by a board of priests from the state religion, but it's more common for it to be ruled by a single king who is also a priest and the head of his priesthood. Such an arrangement may be benevolent, with a wise cleric ruling the land; or it may be particularly nasty, with a power-mad priest or the priest of an evil god in charge.
Many "lost worlds" (i.e., nations hidden away from the rest of the world, secluded in a lost valley or cavern or other distant place) are theocracies ruled by evil priests; adventurer-heroes stumbling into such cultures often find themselves having to stir up revolution and cast down these rulers so that justice can return to these lands.
The State Religion[]
For any culture, the DM will have to decide if there is a State Religion. The State Religion is the official faith of the nation, as sanctioned by the government.
Its priesthood will have the following rights: Right to church trial, right to coronation (though not necessarily the right to deny coronation to the chief candidate), right to demand tithes. If the culture is monotheistic, the State Religion will also have the right to separate offenders from the faith. The DM can assign other rights to the State Religion as he sees fit.
State Religions are not limited to big nations. Any independent city or city-state could have its own state religion. Thus, cities mere miles apart might have different state religions. This could cause trouble if both are monotheistic, or both belong to enemy priesthoods; on the other hand, the priesthoods could be neutral or friendly to one another, causing no such trouble.
However, a nation does not have to have a state religion. In fact, the most powerful faith in a nation will not necessarily be a state religion; it will just exert considerable influence.