Christendom

Christendom refers to those parts of the world, where Christianity was the dominant religion or even the state religion. At one point in time this included the Roman Empire, the Middle East and parts of North and Northeastern Africa. As the Western Roman Empire collapsed Christendom was centered in the Eastern part of the Roman Empire known as Byzantium. As Islam became a religion, Islamic conquest spread to the Middle East, North Africa, and even to Sicily, and Spain. In the Middle East and North Africa the Christian religion retreated as many people converted to Islam.

In Europe, as the invasions of the Northern European Barbarians abated, and many of them converted to Christianity, including the Franks, Germanic Tribes, and ultimately Russia, a period of relative peace developed and cultural and economic development could proceed. Christianity became predominantly a European Religion. As opposed to the Chinese and Islamic civilizations, Europe politically was not a unified Empire. It was comprised of many small political units which were often at odds with each other.

The Byzantine state, in contrast was a centralized state composed of a highly centralized administration controlled by an all-powerful emperor. Its administrators were trained in Greek civilization. While the centralized state created order within its borders, it ultimately succumbed to the Islamic and Western Christian invaders.

In Western Christendom, as opposed to Byzantium, there was a separation between secular and religious authorities.  There were two significant and opposite reactions to Islam in Western Christendom.  In Spain , the contact of Christianity with Islam led  to a rebirth of European connection to Greek civilization  and culture , as well as a connection to Islamic advances in sciences, math, astronomy and philosophy.

The other reaction was war against Islam. There were several crusade in an attempt to retake the Holy land from Islam. In addition, after a prolonged period of war , Islamic rule in Spain ended.   

Photo Reference :
https://www.khanacademy.org/humanities/whp-origins/era-4-regional/42-systems-restructure-betaa/a/read-christendom-beta