Ooh I love this topic!
Manichaeism is the most obvious example of the world power that wasn't, but I think medieval Christian Europe has some interesting examples.
The Irish Book of Invasions for instance, despite being made by Christian monks, actually suggests a radically different theology.
For those unfamiliar, the Book of Invasions details the mythic history of Ireland through the lens of multiple groups coming to the island. Some of these are directly biblical, including Noah's granddaughter (the first settlers), a different group of slaves that escaped during exodus etc.
What is most interesting however is that it has an outright other deity. Not in the sense of the Tuatha and Sidhe, but a full on deity called Danu who bestows the secrets of high magic to the Tuatha.
For someone who believes in the book, the ultimate conclusion must be a kind of Dualism, Danu and Yahweh as two seperate and possibly linked deities, a mother and a father.
Whilst IRL there isn't such a movement that latched on to this, I get very strong vibes of a sort of proto-wicca with some cool trinity theology. Yahweh the father (Yahweh), son (Jesus) and the Holy Ghost (the transcendent) alongside the Mother (Danu), the Daughter (Mary) and the world (Nature). This could perhaps lead to a star of David symbol in this hypothetical religion.
On a similar vein, I have heard and read multiple times (yet never have I found a source for the claim) that Christian missionaries to the Norse would say that Ragnarok had already happened, that the Norse myths were actually stories of the last world and that Baldr, as master of the new world, had a new story in the form of the Bible. Again, I don't know if there is any historical validity to this, but I've seen people make threads about a hypothetical "Norse Muhammad" before and this seems like a perfect fit.