I've mused on metaphysical questions for the early modern period before, but I still haven't stumbled upon something that I like, fits a pseudo-historical interpretation, and is still recognizable with typical D&D tropes. Let's take some inspiration from the Trinity.
There are a ton of denominations and religions throughout the 17th century world and I don't want them ALL to be Clerics or Druids and I probably don't want religion to dictate class when it comes to divine spellcasters. While it would definitely be neat to have a separate class or at least spell list for Catholics, Lutherans, Muslims, Animists, Ancestor Worshipers, Pagans, etc, I think the granularity would mostly be lost in meta-gaming, both in terms of players who WANT certain spells and players who WANT a certain religion making the decision mostly moot. So let's compromise and utilize the Trinity.
God the Father is the omnipotent, uncaring creator of the universe. He created existence with a set of natural laws and is completely above the care of any individual person or world. Followers of God the Father worship "nature" and natural law, either in a tangible sense or as a personification of the Almighty. Spells for these followers are rarely personal and instead invoke upon the power inherent in the laws of creation and existence.
Christ is a personal God incarnate in every living being. Christ is a God who was both divine and mortal and experienced life and death in a finite sense. Followers of Christ retain a personal connection with Him, whom offers personal salvation. Spells for these followers operate on the intrapersonal and interpersonal level to offer either salvation or damnation.
The Holy Spirit is a power manifest in mortal beings who strive for righteousness. He represents power manifest in the self, derived from following the will of the Spirit. The ultimate expression of the Holy Spirit is the attainment of personal divinity, where one becomes part of the gestalt Holy Spirit. Spells for those who worship him are focused on achieving righteous power.
You can probably see where the lines are drawn now. Druids, Rangers, and similar classes/faiths worship God the Father, in one name or another, and use the Druid spell list. Catholics, Christians, and monotheistic faiths follow Christ and use the Cleric spell list. The Holy Spirit is for pagan and polytheistic followers and will need a spell list. One could break all the divine spells into three thematic lists and assign them, adding spells from source books to have a robust selection for everyone.
Either way, this interpretation allows for multiple religions that are all "right" without implying that there is some all-present celestial holy war playing out in heaven and on earth, which is how the Planes have been presented in canon D&D lore. Each individual religion can have its own flavor and doctrines but it derives all power from the same Godhead.
Another thought crosses my mind. What about magic-users? Could they be shoe-horned into that third slot? One could play a Priest of a Pagan religion, or just an atheistic or agnostic magic-user with the same spell list.
Or perhaps magic-users should stay separate.Whereas followers of God the Father pray for spells implicit in natural law, magic-users seek to cast spells that harness, manipulate, and/or contradict natural law.
Fertile ground indeed and I like what has been sown better than previous ideas...