Christendom refers to the mediaeval and renaissance concept of the portion of the world where Christianity holds sway.
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 till Napoleon's defeat of Francis II.
After the collapse of Charlemagne's empire, Christendom became a collection of states loosely connected to the Holy See or to other branches of Christianity, mostly connected to Byzantium. Tensions between the popes and secular rulers ran high, as the pontiffs attempted to retain control over a region that was rapidly breaking into smaller nations. Christendom as a cohesive political unit effectively ended with the Reformation.
