The Orthodox Church and the Roman Catholic Church
A
B
C
D
Were united under the pope, who appointed the patriarch of Constantinople to rule as his representative
Formally split at the same time that the Roman Empire split into the Byzantine Empire and feudal Europe
Worked closely with the Muslim empire to fight against the Mongol invasions of the 13th century
Formally split over doctrine in the 11th century, but still united to fight together in the Crusades