Religious movements in the 6th century

If ethnic hostility within the empire was less a menace around the year 500 than it had often been in the past, dissensions stemming from religious controversy seriously threatened imperial unity, and the political history of the next century cannot be understood without some examination of the so-called Monophysite heresy. It was the second great heresy in the Eastern Empire, the first having been the dispute occasioned by the teachings of the Alexandrian presbyter Arius, who, in an effort to maintain the uniqueness and majesty of God the Father, had taught that he alone had existed from eternity, while God the Son had been created in time. Thanks in part to imperial support, the Arian heresy had persisted throughout the 4th century and was definitively condemned only in 381 with promulgation of the doctrine that Father and Son were of one substance and thus coexistent.

If the Fathers of the 4th century quarreled over the relations between God the Father and God the Son, those of the 5th century faced the problem of defining the relationship of the two natures - the human and the divine - within God the Son, Christ Jesus. The theologians of Alexandria generally held that the divine and human natures were united indistinguishably, whereas those of Antioch taught that two natures coexisted separately in Christ, the latter being "the chosen vessel of the Godhead . . . the man born of Mary." In the course of the 5th century, these two contrasting theological positions became the subject of a struggle for supremacy among the rival sees of Constantinople, Alexandria, and Rome. Nestorius, patriarch of Constantinople in 428, adopted the Antiochene formula, which, in his hands, came to stress the human nature of Christ to the neglect of the divine. His opponents (first the Alexandrian patriarch, Cyril, and later Cyril's followers, Dioscorus and Eutyches) in reaction emphasized the single divine nature of Christ, the result of the Incarnation. Their belief in Monophysitism, or the one nature of Christ as God the Son, became extraordinarily popular throughout the provinces of Egypt and Syria. Rome, in the person of Pope Leo I, declared in contrast for Dyophysitism, a creed teaching that two natures, perfect and perfectly distinct, existed in the single person of Christ.

At the Council of Chalcedon (451), the latter view triumphed thanks to the support of Constantinople, which changed its position and condemned both Nestorianism, or the emphasis on the human nature of Christ, and Monophysitism, or the belief in the single divine nature.

More important for the purposes of military and political history than the theological details of the conflict was the impact Monophysitism produced on the several regions of the Mediterranean world. Partly because it provided a formula to express resistance to Constantinople's imperial rule, Monophysitism persisted in Egypt and Syria. Until these two provinces were lost to Islam in the 7th century, each Eastern emperor had somehow to cope with their separatist tendencies as expressed in the heresy. He had either to take arms against Monophysitism and attempt to extirpate it by force, to formulate a creed that would somehow blend it with Dyophysitism, or frankly to adopt the heresy as his own belief. None of these three alternatives proved successful, and religious hostility was not the least of the disaffections that led Egypt and Syria to yield, rather readily, to the Arab conqueror. If ever the East Roman emperor was to reassert his authority in the West, he necessarily had to discover a formula that would satisfy Western orthodoxy while not alienating Eastern Monophysitism.