The Roman Catholic Church is the largest Christian church, with approximately 1.3 billion baptised Catholics worldwide as of 2018.

In 1095, Byzantine emperor Alexius I appealed to Pope Urban II for help against renewed Muslim invasions in the Byzantine–Seljuk Wars, which caused Urban to launch the First Crusade aimed at aiding the Byzantine Empire and returning the Holy Land to Christian control. In the 11th century, strained relations between the primarily Greek church and the Latin Church separated them in the East–West Schism, partially due to conflicts over papal authority. The Fourth Crusade and the sacking of Constantinople by renegade crusaders proved the final breach. In this age great gothic cathedrals in France were an expression of popular pride in the Christian faith.

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