The seeds of Christendom were laid in 306 A.D., 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, 800 A.D., 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.