If the fair world equivalent of Christianity is the Carpenter Cult, what is the main religion of the setting?
It’s a disorganized pantheistic sort of belief, with most people offering devotions (rituals, service, etc.) chiefly to one god but acknowledging others, and a few people offering devotions to none. Even in smaller communities, numerous gods tend to be represented by temples, shrines, and priests, but they do tend to be grouped by the ethnic origins of the local population. An area settled by the descendants of Celtic equivalents will tend to follow Celtic gods, one settled by the descendants of Greek equivalents will tend to follow Greek gods, etc.. There is a general though unsystematic belief that one’s god is known elsewhere by other names and with other attributes. The Carpenter Cult is unusual in that it encourages its adherents to believe that only Yahweh is a true god, with all the other divinities being of lesser orders.