[Because I'm always tinkering with the D&D rules, here is a part of an effort I had made in 2007 to micro-manage level advancement for character classes in a different way from the published rules, along with some information specific to a campaign setting I was toying with creating at the time.]
Those who pursue power through faith are clerics. These are fighting men who have sworn special oaths to the gods, which in turn allows them to call on the power of the gods. This power may be manifested in spell-like miracles, or control over the undead. Clerics are organized, and generally Lawful in nature (see Alignment). Their church is hierarchical, so that a cleric advances through ranks of increasing importance.
Wisdom is the prime requisite for clerics and they gain benefits for having high Wisdom scores (see Ability Scores).
The demi-human races may be clerics, but require three times as much xp as shown below to advance in level.
Clerics are decreasingly restricted in the use of armor and weapons as they rise in level, though they can never use edged magic weapons (i.e., any bladed or pointed weapon). Clerics of even the most vile and despicable nature must avoid this taboo, for its origin hails from an ancient pact the gods made after a great war.
After a cleric has proven he is worthy of becoming an acolyte, he may petition the gods for spells. Divine magic does not work in quite the same manner as a magic-user casts spells. The cleric must only be able to speak out loud and give a short speech beseeching divine aid. There will always be some manifestation of presence associated with the spell. For first level spells, this could be a dim glow, a moving shadow, or a startling breeze.
If the cleric has the option of petitioning multiple gods, they cannot be diametrically opposed in Alignment.
The undead are particularly vulnerable to the presence of the divine. All clerics can affect them, but with different results depending on Alignment. In any case, when turning undead, clerics roll 2d6. A result occurs if the result is the same or higher than the number needed (shown below. A successful turning lasts for 3-12 rounds, and affects 2-12 undead monsters. A cleric can only attempt to turn an individual undead monster once per week. If a turned undead monster is attacked by anyone, the turning is broken and the undead may attack back.
If the cleric is Lawful, the undead are forced to flee. Turned undead will move away from the cleric at full movement rate until cornered or time has elapsed.
If the cleric is Neutral, the undead will ignore the cleric as if he was not there.
If the cleric is Chaotic, the undead will obey a simple one-sentence command.
Title: Exorcist XP: 0-1,250
HD: 1d6 Fights As: 1st level
Special Abilities: The first mystery of the church shown to the newly ordained cleric is how to invoke the names of the potentates of the church, such as the patriarch Milula, to summon divine power. Focused through a holy item, the exorcist can use this power to make weaker forms of the undead recoil, or turn. Your DM will know your chance of undead turning (See Monsters & Treasure).
Exorcists are not entrusted with miraculous prayers (i.e., spells) by the church, but if they possess exceptional Wisdom they may have the ability to invoke them anyway (bonus spells are applicable, even though clerics do not officially start gaining spells until 2nd level). The bonus spells must be chosen from the start from the cleric spell list and cannot later be changed. Each spell may be cast once per 24 hours, during nighttime.
Spells: 0 (but see above)
Exorcists can read and write in one of the languages they can speak (plus more if Intelligence is high or exceptional).
An exorcist can request up to 90 gp worth of purchases (including items, animals, and hirelings) from the church before undertaking a quest.
Limitations: Exorcists may wear padded, leather, or scalemail armor (armor of chainmail or better is forbidden). They may use shields and helms. They may fight only with a weapon that normally has neither blade nor point.
They are expected to tithe 90% of all wealth gained or earned to the church.
Obligations: Exoricists are defenders of the local congregation from the spirit worlds. An exorcist will also have duties at a shrine or chapel that could be as mundane as singing in a choir, tending a garden, or writing documents, but most likely will involve guard duty. Duties will be excused during church-approved quests.
Title: Acolyte XP: 1,251-2,500
HD: +1d6 Fights As: 2nd level
Special Abilities: As an exorcist, but the acolyte may call on a local god, like Raglala of the Whitewood; a minor divinity, such as the angel Ulnemne; or a high prophet of the church, such as Anna Lagnemne, for divine spells. The acolyte can pray for a new 1st level clerical spell (plus possible bonus spells for high Wisdom) once every 24-day and can cast it (them) from the beginning of dusk to the end of dawn.
Spells: 1 1st level
An acolyte may request up to 180 gp worth of purchases before a quest.
Limitations: In battle, acolytes may wear armor up through chainmail, but are still limited to blunt weapons.
Acolytes must tithe 80% of all accumulated wealth.
Obligations: An acolyte serves in a shrine or chapel, answering questions from lay visitors, tending to the needs of the faithful, and assisting with services when not on quests.
Title: Subdeacon XP: 2,501-5,000
HD: +1d6 Fights As: 2nd level
Special Abilities: As per an acolyte, except as follows. A subdeacon may call on a demigod for their divine spells, such as Izapisne, Izumud, and Rudemne, or any demigod so long as it is recognized by his church and not of an Alignment that runs counter to the general behavior of the subdeacon. The subdeacon can pray for spells once every 22 hours and can cast spells up to one hour before dusk and one hour after dawn.
Spells: 2 1st level
Subdeacons can read and write in one more language they can speak.
A subdeacon can request up to 360 gp in purchases before a quest.
Limitations: Subdeacons must begin choosing a permanent Alignment, and can only call on deities of a similar Alignment from then on without serious repercussions (left to the DM, but should include loss of all clerical powers). For example, if a subdeacon always obeyed mortal laws, a Chaotic demigod would not answer his prayers.
In battle, subdeacons may wear up to heavy splintmail armor. They may wield any blunt weapon and only one bladed/edged nonmagical melee weapon (chosen upon reaching 3rd level).
Subdeacons must give 70% of all wealth to the church.
Obligations: Subdeacons help with the administrative duties of a shrine or chapel. They may preach to the local lay people or conduct other services tasks as needed.
Title: Deacon XP: 5,001-10,000
HD: +1d6 Fights As: 3rd level
Special Abilities: As sub-deacon, but deacons may cast 2nd level cleric spells by invoking a demigod of his Alignment. The deacon can pray for spells once every 20 hours and can cast spells up to two hours before dusk and two hours after dawn.
2 1st level 1 2nd level
A deacon can request up to 700 gp in purchases before a quest.
Deacons can wear any nonmagical armor without restrictions.
Limitations: Deacons must act of one Alignment. A pattern of transgressions will be answered by no deities answering prayers for spells (read as no spellcasting ability) until the deacon has in some way atoned (as detailed by the DM).
A deacon may choose one additional bladed/edged nonmagical weapon, either for melee or hurling (such as an axe).
Deacons must tithe 60% of all wealth to the church.
Obligations: The lowest in the hierarchy of church rulers, deacons are in charge of shrines, are personally responsible for the preaching to and conversion of non-natives in their region, and oversee the exorcists, acolytes, and subdeacons in a parish.
The druids worship the old gods, primitive deities such as Dapanna-Abadin, Mitanikala, Ruag, and Ututrabu, who ask blood sacrifice of their followers. They advance in level just like a cleric, but gain none of the special abilities past 1st level, except for spell casting. They suffer none of the limitations and obligations of the cleric class.