Most fanfiction writers are probably secular, or nominally Christian, or vaguely spiritual. They are not consciously reinventing paganism. They are not sitting down and saying: "I shall now construct a pagan theological framework to undergird my pseudo-feudal narrative."
But the structural logic of the system they are building compels them toward paganism. Every choice they make — the blood oath, the magical contract, the Lord ring, the ancestral magic, the ranked cores, the house-elf, the betrothal agreement — requires a specific kind of divine authority to sustain it. And that authority can only be pagan.
Because:
If the ultimate authority is moral, it will condemn the hierarchy. → So the authority must be amoral. → Pagan.
If the ultimate authority is universal, it will equalize all magical beings. → So the authority must be partisan. → Pagan.
If the ultimate authority is transcendent, it will stand outside the system and judge it. → So the authority must be immanent. → Pagan.
If the ultimate authority is gracious, it will forgive the oath-breaker and liberate the bound. → So the authority must be transactional. → Pagan.
If the ultimate authority is ethical, it will favor faith over blood. → So the authority must be genealogical. → Pagan.
If the ultimate authority is personal, it will relate to each individual soul, and the least vassal will be as visible as the greatest lord. → So the authority must be impersonal, a force rather than a person. → Pagan.
Every axis of the Christian God points against the system. Every axis of the pagan gods points toward it. The writer who wants the system must choose the pagan model, whether they know it or not.