Through the actions of the Roman emperors Constantine and Theodosius (see "The Acceptance of Christianity" in Chapter 7), Christianity became in some ways a state as well as a religion. Early medieval writers began to use the word Christendom to refer to this Christian realm. When the pope called for holy war against the Muslims, for example, he spoke not only of the retaking of Jerusalem, but also of the defense of Christendom. When missionaries, officials, and soldiers took Christianity into pagan regions, they understood their actions to be aimed at the expansion of Christendom.

From the point of view of popes such as Gregory VII and Innocent III, Christendom was a unified hierarchy with the papacy at the top. They pushed for uniformity of religious worship and campaigned continually for use of the same religious service: the Roman liturgy in Latin, in all countries and places. As we have seen in this chapter, however, not everyone had the same view. Kings and emperors may have accepted the Roman liturgy in their lands, but they had their own ideas of the way power should operate in Christendom, even if this brought them into conflict with the papacy. They remained loyal to Christendom as a concept, but they had a profoundly different idea about how it should be structured and who could best defend it. The battles in the High Middle Ages between popes and kings and between Christians and Muslims were signs of how deeply religion had replaced tribal, political, and ethnic structures as the essence of Western culture.