From Academic Kids
Christendom, in the widest sense, refers to Christianity as a territorial phenomenon: those countries where most people are Christians, or nominal Christians, are part of Christendom.
Christendom as a polity
In a more significant and meaningful sense, it refers to the medieval and renaissance notion of the Christian world as a sort of social and political polity. In essence, the vision of Christendom is a vision of a Christian theocracy, a government devoted to the enforcement of Christian values, and whose institutions are suffused with Christian piety. In this vision, members of the Christian clergy wield plenty of political clout. Secular rulers are their subordinates and agents; and national or political divisions are subsumed under the unitary government of a unique and universal church institution. This tempting vision of an earthly crown was one of the greatest challenges to the institutional Christian church.
The seeds of Christendom were laid in A.D. 306, when Emperor Constantine became co-ruler of the Roman Empire. In 312 he converted to Christianity, and in 325 Christianity became the official religion of the Empire.
Christendom was given a firmer meaning with the creation of Charlemagne's kingdom, the Christian Empire of the West. On Christmas Day, A.D. 800, Charlemagne was crowned by the Pope as ruler of the Holy Roman Empire, a title which would exist up until Napoleon's defeat of Francis II in 1806.
After the collapse of Charlemagne's empire, Christendom became a collection of states loosely connected to the Holy See. Tensions between the popes and secular rulers ran high, as the pontiffs attempted to retain control over their temporal counterparts. The idea of Christendom was already greatly discredited by the time of the Rennaissance Popes because of the moral laxity of the pontiffs and their willingness to make war, peace, and alliances like secular rulers.
Christendom as a cohesive political unit effectively ended with the Reformation. Christendom can also refer to Christians considered as a group: The Christian World.
There is another sense to the polity, with less of a secular meaning, which would have been compatible with the idea of both a religious and a temporal body: Corpus Christianum.
The Latin term Corpus Christianum is often translated as the Christian body, meaning the community of all Christians.
It described the pre-modern notion of the community of all Christians united under the Catholic Church. This community was to be guided by Christian values in its politics, economics and social life. Its legal basis was the corpus iuris canonica (body of canon law). The Church's overarching authority over all European Christians in the Middle Ages and common endeavours of the Christian community -- for example, the Crusades, the fight against Moors in Spain and that against the Ottomans in the Balkans -- helped to develop this sense of communal identity against the obstacle of Europe's deep political divisions. The Corpus Christianum can be seen as a Christian equivalent of the Muslim Ummah. The concept also justified the Inquisition and anti-Jewish pogroms, to root out divergent elements and create a religiously uniform community.
This concept has been in crisis since the late Middle Ages, when the kings of France managed to establish a French national church during the 14th century and the papacy became ever more aligned with the Holy Roman Empire of the German Nation. The Empire, due to its massive size, did represent a large portion of European Christians. Thus the Corpus Christianum was limited to the Christian community of the Empire, rather than all Christians worldwide.
The rise of Modernity and the Reformation during the early 16th century deconstructed the Corpus Christianum even more. The acceptance of different interpretations of the Bible by the Peace of Augsburg in 1555 officially ended the idea that all Christians could be united under one church. The principle of cuius regio, eius religio ("whose the region is, his religion") established the religious, political and geographic divisions of Christianity. The Corpus Christianum was replaced by something foreshadowing the modern idea of a tolerant and diverse society consisting of many different communities.