When contemplating the idea of running a Pathfinder game set, loosely, in the real world in an early-modern setting, I quickly came up against the issue of how to handle divine spellcasting. One of the cool things about Clerics in Pathfinder is the wide range of domains available to them and the myriad combinations of abilities those provide. While I could have hand-waived it and allowed "Christian" clerics access to any spells they wished, or made up complex tables of which domains were allowed to which religions, I decided instead to re-think how the existence of clerics and magic might have alternately shaped history...
Because, seriously, there are people casting spells...
Disclaimer, I do speak about real-world religions here, but this is all meant as setting for a Fantasy Earth. You are free to get offended if you wish. You are free to read into this some real insight about those religions if you see such. Just don't think that this in any way reflects my own belief system...
The Church of Cultural Appropriation
One religion has dominated the entire history of the western world (no, not the "Abrahamic faiths")... I'm talking about Cultural Appropriation.
The Egyptians, the Greeks, the Romans -- all of these shared a tendency, not to enforce their religion on others, but rather to adopt the gods of their neighbors, their trade partners, and even their conquered enemies as their own. Not only did these ancient cultures have vast pantheons of gods, both major and minor, but each god had numerous aspects, numerous names, both local and foreign. Saint Paul is famously quoted in his address to the philosophers of the Areopagus (the Hill of Mars) in Athens: “Men of Athens, I perceive that in all things you are very religious; for as I was passing through and considering the objects of your worship, I even found an altar with this inscription: TO THE UNKNOWN GOD. Therefore, the one whom you worship without knowing, him I proclaim to you..."
With the rise of Christianity, this tendency to adopt and incorporate foreign gods did not go away. The Catholic Church especially (which literally means "All Embracing"), after it became the official state religion of Rome, in its efforts to "convert" various pagan sects did not deny them the right to pray to their gods, but instead rebranded them as Saints and incorporated them into the orthodoxy: Saint Brigid, Saint Ador Ormazd ("Ahura Mazda") in Syriac churches, and many others. Eventually reforms happened and the adopted saints of the Christian church were cast out (or quietly downgraded) to make room for a stricter form of monotheism, but, even then, the Apostoles are spoken of with reverence and the "One God" is referred to as a triune-being.
The same is true in the East as well. Mongolian shamanism manages to simultaneously embrace, not only those spirits unique to the Mongolian culture, but also the worship of the Buddha and Ahura Mazda (the god of Zoroastrianism). Traditional Han Chinese folk religion incorporates the teachings of Taoism, Buddhism, and Confucianism, alongside thousands of gods, spirits, national deities, cultural heroes, and ancestor spirits.
Imagine then, a world where the servants of the gods can cast spells. Where clerics are real, and the spheres and domains of the gods are made manifest through mortal servants...
"Your god is a false god!"
"Really? I don't think so. Because Bob over there just prayed to Dionysius and made food out of thin air..."
"Oh...well shit. Well, at least let me introduce you to this new god of mine whom you might not already be praying to..."
Imagine Moshe standing before Pharaoh, performing miracles. Even in the official narrative, Pharaoh's "sorcerers" can match the first few of these, but in this world it does not stop there. Or Elijah's contest against the Baalites to see who could call down fire to light an altar....but they both succeed. If spells are real, and the spokesman for each god can perform wondrous acts, and cultural appropriation is the norm, there is no place for monotheism to take a foothold.
In the world of Beyond the Sea, the gods are many, and the gods have power, just not much of it. Classic D&D lore (s.p. the Forgotten Realms) suggests that a god's power is tied to the number and fervor of their worship. If clerical magic is the norm, and pantheism is the norm, and cultural appropriation is the norm, then no one god will ever gain sufficient focused worship to elevate himself above the others. If a 'One God' is never able to establish a strict monotheocracy, how would he amass the power necessary to be 'One'?
Imagine now, a world where the Israelites were never freed from Egypt, but never really enslaved either...they, and their god, were simply assimilated into the Egyptian culture and religion. A long line of prophets served the God of Israel, but other prophets performed equally flashy miracles in the name of their gods--Thoth, Ahura Mazda, Pan, Baldr, the Jade Emperor, et. al. Later, a prophet calling himself The Christ rose up in the land of Palestine under Roman occupation, and even his revolutionary teachings were assimilated by the Greek philosophers, becoming just another demi-god in a very long line of such after his death.
To make things harder for would-be monotheists, there is the tendency for so many of everyone's ancestors to come back and stick around for an extra-long time. Stories of eternal rewards or eternal punishments have little meat to them when your great-great-great-grandmother who died at sea is still around two centuries later managing the family and dispensing wisdom.
In the world of Beyond the Sea, civilizations have risen and fallen in pretty much the same way as in our world, but monotheism just never took off. The great empire of Rome fell, but the ideals of the "All Embracing (Catholic) Church" and its worship of the innumerable gods, demi-gods, saints, lares, jumped-up rulers, prophets, demons, ancestral spirits, and celestial bureaucrats lived on, and the altars to "The Unknown Gods" stand in every town.
There Can be Only One:
For each of the many, many, many gods of the world of Beyond the Sea there is but one cleric (each). They may have a great many worshipers, lay and ordained, but at any given time, only one truly devout prophet bears the torch of the god and wields his/her/its divine might in the world. There was, after all, only one living Oracle of Apollo at Delphi at a time and only ever one prophet in Israel at a given time (though in both of these cases, there was usually a student or apprentice ready to take their place).
If the gods draw their power from worship, and all worship is diluted by the pantheist tendencies of the All-Embracing Church and the sheer multitude of gods known to every would-be worshiper, then the power of the gods is scarce. Even with scarce power at their disposal, the gods must show their power in the mortal world, or else risk vanishing entirely. Thus, each god, has but one servant able to cast spells, and even these, most devout of servants, are likely to pray to other gods when the situation would call for it.
Clerics, Druids, Inquisitors, and similar classes always worship their own single, specific god. Indeed, no god grants spells to more than a single faithful servant (none is so powerful as to do more). These priestly characters may be of any alignment, and may choose any domains they wish--even alignment domains opposed to their own (there may be a god of good men who have evil thoughts for example). Those that gain proficiency with a "deity's weapon of choice" may choose any single martial weapon of their choice. Players are encouraged to make up the details of their chosen god.