Through the actions of the Roman emperors Constantine and Theodosius (see 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. Sometimes notions of Christendom were linked directly to specific states, such as Charlemagne’s empire and the Holy Roman Empire. More often, however, Christendom was vague, a sort of loose sense of the body of all people who were Christian. 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 the Iberian Peninsula, Scandinavia, or the Baltic region, they understood their actions as 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. They forbade vernacular Christian rituals or those that differed in their pattern of worship. Under Innocent III papal directives and papal legates flowed to all parts of Europe. Twelve hundred church officials obediently came to Rome from the borderlands as well as the heartland for the Fourth Lateran Council of 1215.

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.