During the first millennium and a half the classical Christian tradition developed its distinctive ritual, church structures, and theology. What had begun as a Jewish sectarian movement in Palestine moved into the wider world of the Roman Empire, combining aspects of Judaism and Greco-Roman culture.  That was the first great cultural amalgam. In the fourth century, in one of the great ironies of history, Christianity became the established religion of the Empire that had executed Jesus and persecuted his followers. Christendom was founded on that irony.

A second cultural amalgam occurred during the last 500 years of the first millennium; the conversion of Celtic, Germanic, Slavic, and other peoples produced a synthesis of classical Greco-Roman civilization and the invading cultures.  That synthesis shaped medieval Christianity. From the beginning,  the Christian tradition has been multi-cultural.  Judaism and Greco-Roman Culture produced classical Christianity; classical Christianity and “barbarian” cultures [Celtic, Germanic and Slavic] produced  medieval Christianity.

Christendom, the social, cultural and political embodiment of Christianity, although preserving the ideal of a unified Roman Empire,  developed into distinct Latin and Greek traditions. Between 1000 and 1500 the schism was completed and the two cultural traditions of Christendom were established, one centered in the old Rome and the other in the new Rome, Constantinople.

In the first 500 years of the second millennium the Western  and Eastern variations of classical Christianity and Christendom came into full flower, shaping the development of western civilization up to the modern period.