> Would a group be able to call itself Christian, while not believing in a single divine god and that Jesus was his prophet?

That was the whole debate over Arianism.

You can see this same sort of tension in the Torah too, specifically in the Deuteronomistic account. There's all sorts of remarks about those dang southern Jews and their heretical paganism. We also know of the apparently polytheistic Elephantine Jews from archaeological/literary evidence.

Let's not forget that monotheism and the impossibility of other gods is also a particularly Abrahamic belief that's not widely shared in other ancient traditions.