Yesterday (Piety Police and Karma Cops), I laid out some notes culled from the AD&D Dungeon Master's Guide and Deities and Demigods Cyclopedia on how the DM was advised to manage clerics back in the early days: the DM should establish the tenets of the cleric's deity, and the cleric's ability to get higher level spells (or atone for transgressions) would be subjectively determined by the DM based on how closely the player adhered.
I'm a bit surprised the concept of "judging your cleric's behavior" doesn't get more discussion; it makes me about as uncomfortable as dissecting behavior for determining alignment. Do folks ignore clerics and their gods? Build frivolous deities like Pabst the Beer god and just let the cleric exist as a heal-bot? (I've laid out the argument for a "godless" cleric when we do a future campaign to cleave closer to the ambiguous cosmology in most literary weird horror).
The backdrop for Gothic Greyhawk is supernatural horror, and postulates a strong monotheistic church and an ancient conflict between Law and Chaos, and to a lesser extent good versus evil. Elements of the cosmology were laid out in previous posts on cosmology, Law, and Chaos, and the monotheistic deity was identified as Pholtus from the original Greyhawk campaign. With that in mind, here's the approach I'll be proposing for clerical beliefs in Gothic Greyhawk.
Spell casting clerics are a different breed from trained clergy. Trained clergy run churches, temples, and monasteries and perform a similar role as in the (early) medieval world - scholars, protectors of knowledge, hospitalers, and theologians.
Spell casting clerics are chosen by supernatural fiat and come from a wide range of backgrounds. A humble farmer might have an angelic or supernatural visitation and suddenly manifest the ability to do clerical magic. Many of these people are taken in by the church and formally trained, and most of the highest ranks in the church hierarchy are filled by spell casters. However, some clerical spell casters never join the church, and remain outside as prophets, mystics, reformers, and heretics.
In this way, a player can be a cleric with or without formal church training while they figure out the direction of their character. It provides an interesting opportunity to have a "reluctant cleric". Also, using the Bible as a mythic source, most of the spell casting / miracle worker types didn't really ask for a special destiny, they were chosen.
Higher Level Spells
3rd level spells and higher are bestowed by servants of the deity. Once the cleric achieves enough experience to cast 3rd level spells, they need to reach out through prayer to these supernatural agents. In Deities and Demigods these are listed as angels and demigods, though I'll include saints in lieu of demigods to reinforce the medieval pseudo-Christian theme.
The gaining of 3rd level spells will be the first point the cleric's adherence to the tenets of the church will matter (mechanically) - somewhere around 5th level for most editions.
Tenets of the Church
I'm going to keep the tenets of the church fairly high level and straightforward from a behavioral standpoint; as I mentioned, I think it's important for the campaign theme that the church have meaning and stand for something, but I'm not interested in micro-managing player behavior, either.
The two monotheistic churches of Pholtus (there was a schism) stand for the twin virtues of compassion and justice; the Church of the Blinding Light in the Theocracy of the Pale focuses more on the justice side of things, the Church of the Eternal Spirit (in Veluna) emphasizes the compassionate side of the creator. Compassion and justice are slippery enough (taken together) that a clever player should be able to align most of his activities with furthering one or the other, as long as it's for the greater good.
Both churches emphasize the need to liberate humanity from pagan gods, nature spirits, devils, demons, and the fairies of Chaos - all supernatural entities that lead people into false worship (according to the church). From the DM's perspective, this ensures there's a healthy amount of conflict between believers of Pholtus, and everybody else - no shortage of bad guys.
Finally, the teachings of the church posit an afterlife in Pholtus's heaven that requires both the god's mercy and a pure soul; engaging in pilgrimages, religious observances such as feast days and rites, and even military service (crusades) are ways for believers to work off the burden of bad behavior - once again reinforcing the medieval themes.