Christendom usually refers to Christianity as a territorial phenomenon. It can also refer to the part of the world in which Christianity prevails.
The term Christendom has been used to refer 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, whose institutions are suffused with Christian doctrine. In this vision, members of the Christian clergy wield political authority. The specific relationship between the political leaders and the clergy can vary but, in theory, national or political divisions are subsumed under the leadership of a church institution. This vision would tempt Church leaders and political leaders alike throughout European history.
Emperor Constantine issued the Edict of Milan in 313 proclaiming toleration for the Christian religion, and convoked the First Council of Nicaea in 325 whose Nicene Creed included belief in "one holy catholic and apostolic Church". Christianity became the state religion of the Empire in 392 when Theodosius I prohibited the practice of pagan religions. The Church gradually became a defining institution of the Empire.
As the Western Roman Empire disintegrated into feudal kingdoms and principalities, the concept of Christendom became less defined in the West and the Christians of the Eastern Roman Empire (and the subsequent Byzantine Empire) came to see themselves as the last bastion of Christendom. The vision would eventually take a radical turn with the rise of the Franks, a Germanic tribe that converted to the Christian faith and entered into communion with Rome. On Christmas day 800 AD, Pope Leo III made the fateful decision to switch his allegiance from the emperors in Constantinople and crowned Charlemagne, the king of the Franks, as the Emperor of what came to be known as the Holy Roman Empire. This empire created a competing definition of Christendom in contrast to the Byzantine Empire. The question of what constituted true Christendom would occupy political and religious leaders for centuries.
After the collapse of Charlemagne's empire, the Holy Roman Empire became a collection of states loosely connected to the Holy See of Rome. Tensions between the Popes and secular rulers ran high, as the pontiffs attempted to exert control over their temporal counterparts and vice versa. The idea of Christendom in the West was already greatly discredited by the time of the Renaissance Popes because of the moral laxity of the pontiffs and their willingness to seek and rely on temporal power as secular rulers did.
In the East, Christendom became increasingly well defined as the Byzantine Empire's gradual loss of territory to an expanding Islam caused Christianity to become ever more important to Byzantine identity. Even after the East-West Schism which divided the Church, there had always been a vague notion of a universal Christendom that included the East and the West. The Byzantines would be divided among the Byzantine rite of the Orthodox Church and the Byzantine rite of the Catholic Church. The political unity (now without religious unity) was finally destroyed by the Fourth Crusade when Western Christian mercenaries conquered the Byzantine capital of Constantinople and set the Byzantine Empire on the path to annihilation.
With the breakup of the Byzantine Empire into individual nations with nationalist Orthodox Churches, the term Christendom became more valid in describing Western Europe, Catholicism and the non-Orthodox among Byzantines and other Eastern rites of the Church.
The Reformation and the ensuing decline and breakup of the Holy Roman Empire into independent states caused the term "Christendom" to take on a more informal meaning in Western Europe signifying countries which were predominantly Christian as opposed to Islamic or pagan countries.
Catholics who advocate Christendom's restoration argue that, with the division of Protestantism into many denominations, Christendom could only apply to the civilization of Catholic nations that espoused the doctrine of the Social Reign of Christ the King.
The term can also refer to Christians considered as a group (the "Christian World") or to the informal cultural hegemony that Christianity has traditionally enjoyed in the West.
It described the pre-modern notion of the community of all Christians united under the Roman 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 peak of authority over all European Christians in the Middle Ages and common endeavors of the Christian community — for example, the Crusades, the fight against the Moors in the Iberian Peninsula 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. Other developments in philosophy and events in England and Europe were also critical: the War of the Roses, the Hundred Years' War, the end of feudalism and the rise of strong, centralized monarchies presaging the modern nation-state. 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 Reformation and rise of modernity in the early 16th century entailed the further deconstruction of the Corpus Christianum. 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 religion ("whose the region is, his religion") established the religious, political and geographic divisions of Christianity, and this was established in international law with the Treaty of Westphalia in 1648, which finally legally ended the concept of Christian unity, i.e. the "One, Holy, Catholic, and Apostolic Church" of the Nicene Creed. With the Treaty of Westphalia, the Wars of Religion came to an end, and in the Treaty of Utrecht of 1713 the concept of the sovereign national state was born. The Corpus Christianum was replaced by something foreshadowing the modern idea of a tolerant and diverse society consisting of many different communities.