Religious authority and the state were conceived very differently in the Byzantine world, the Muslim world, and Latin Europe. This last was the only civilization in which the age-old and nearly universal notion of sacred monarchy was consistently and increasingly challenged. In the Byzantine and Muslim worlds (except among the Shīʿites), the ruler remained a focal point of living religious authority. The Latin West was also distinguished by the variety of views expressed on this subject and the variety of forms which the relationship between church and state took. In some places the bishop was also temporal ruler, whereas in self-governing cities clergy had no overt political role. In feudal kingdoms, bishops were major landowners and often state counselors, but they owed their position partly to royal favor. Moreover, the way in which the church–state relationship was understood and theorized in the West changed as time went by. In Byzantium, by contrast, it never changed, while in the Muslim world, changes were rarely registered in what people wrote down.
Eastern Christendom drew its notion of sacred monarchy from Roman imperial ideology, christianized by Eusebius of Caesarea (ca. 260–340). The one emperor was supposed to reflect the one God; he ruled “by God’s favor” as “God’s deputy.” The common form of address to a Byzantine emperor was “O most divine emperor.” For more than a thousand years, the Eastern church regarded the empire as an essential part of the expression of Christ in the world.