In AD 431, the Council of Ephesus (the Third Ecumenical Council) had condemned the teachings of Nestorius that asserted that Christ was separate as man and divine. After the council, unrest still remained between the Patriarchs of Alexandria and Antioch, Cyril and John respectively. Cyril accused John of maintaining beliefs in Nestorianism. John argued that Cyril held to Apollinarism, another heretical belief that Christ did not possess a human mind and was entirely divine in all but body. Many believed this was an overcompensation for the earlier Arian belief system which asserted that Christ was not divine in any respect whatsoever. Their differences were eventually settled with the moderation of Bishop Acacius of Beroea.
Two years after the death of Cyril in AD 444, Eutyches, a monk of Constantinople began to spread teachings that he perceived as similar to Cyril’s original interpretation of the nature of Christ (that being one nature) to combat another uprising of Nestorianism in the Byzantine empire. However, his teachings were regarded as Docetism, a belief system that effectively denied the human nature of Christ and believed him to be entirely divine, regarded as the exact opposite of Arianism. Though many, including Pope Leo I, interpreted Eutyches to spread untrue teachings more out of a lack of base knowledge than out of any attempt to supplant Nicene Christianity and its belief in Christ as both man and divine, Eutyches’ influence proved surprisingly great. He was branded a heretic in AD 448 and called for his removal from office. However, due to his popularity among the people, Emperor Theodosius II personally opposed this due to Eutyches later repenting.
However, this did not quell controversy between the Patriarchs of Constantinople and Alexandria. A second Council of Ephesus was called in 449, with Pope Leo invited to preside. The pope declined, but sent representatives and documents outlining the Papacy’s position on the matter. He wrote that Christ was indeed of two natures, both human and divine. This definition conflicted with Cyril’s Twelve Anathemas that had been put forth years before. Studies to determine the compatibility between “Leo’s Tome” and the Twelve Anathemas took place leading up to the Second Council of Ephesus.
However, Leo’s Tome would not be addressed at this council, and the presiding Pope Dioscorus of Alexandria, in a series of events that left representatives beaten and threatened, declared that the Twelve Anathemas were orthodox, maintaining that Christ originally had two natures that merged into one nature after incarnation. When Pope Leo received news of this, he declared it a “synod of robbers” and ignored any and all of its pronouncements as a result. The conflict threatened a great schism between Western and Eastern belief.
Despite Leo’s insistence to Emperor Theodosius to reconvene, the emperor continued to install supporters of Dioscorus up until his dramatic death, whereupon an orthodox Christian named Marcian assumed the throne. He agreed with Leo to reconvene, insisting it take place in Nicaea, though it needed to be relocated (due to invading Huns) to Chalcedon in AD 451.
This Council of Chalcedon discussed with finality the matter of the Incarnation and the nature of Christ. The council came to agreement that Leo’s pronouncement was in accordance with Cyril’s teachings, and would be used as orthodoxy. Christ was recognized with two natures, both of man and of divine. This would come to be known as the Chalcedonian Definition.