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The Definition of Chalcedon was supplemented by two later councils, both held at Constantinople. The fifth Ecumenical Council (553) reinterpreted the decrees of Chalcedon from an Alexandrian point of view, and-sought to explain, in more constructive terms than Chalcedon had used, how the two natures of Christ unite to form a single person. The sixth Ecumenical Council (680-81) condemned the heresy of the Monothelites, who argued that although Christ has two natures, yet since He is a single person, He has only one will. The Council replied that if He has two natures, then He must also have two wills. The Monothelites, it was felt, impaired the fullness of Christ's humanity, since human nature without a human will would be incomplete, a mere abstraction. Since Christ is true man as well as true God, He must have a human as well as a divine will.
During the fifty years before the meeting of the sixth Council, Byzantium was faced with a sudden and alarming development: the rise of Islam. The most striking fact about Muslim expansion was its speed. When the Prophet died in 632 his authority scarcely extended beyond the Hejaz. But within fifteen years his Arab followers had taken Syria, Palestine, and
Egypt; within fifty they were at the walls of Constantinople and almost captured the city; within a hundred they had swept across North Africa, advanced through Spain, and forced western Europe to fight for its life at the Battle of Poitiers. The Arab inasions have been called 'a centrifugal explosion, driving in every direction small bodies of mounted raiders in quest of food, plunder, and conquest. The old empires were in no state to resist them." Christendom survived, but only with difficulty. The Byzantines lost their eastern possessions, and the three Patriarchates of Alexandria, Antioch, and Jerusalem passed under infidel control; within the Christian Empire of the East, the Patriarchate of Constantinople was now without rival. Henceforward Byzantium was never free for very long from Muslim attacks, and although it held out for eight centuries more, yet in the end it succumbed.