Council of Chalcedon

===Eutychian controversy===
{{Main|Eutyches}}
About two years after [[Cyril of Alexandria]]'s death in 444, an aged [[monk]] from Constantinople named [[Eutyches]] began teaching a subtle variation on the traditional [[Christology]] in an attempt to stop what he saw as a new outbreak of [[Nestorianism]].<ref name=EB>{{Cite web|url=https://www.britannica.com/biography/Eutyches|title=Eutyches &#124; Biography, Eutychianism, Beliefs, & Facts|website=Encyclopedia Britannica}}</ref> He claimed to be a faithful follower of Cyril's teaching, which was declared orthodox in the Union of 433.

Cyril had taught that "There is only one ''physis'', since it is the Incarnation, of God the Word." Cyril apparently thought that the [[Greek language|Greek]] word ''physis'' meant approximately what the [[Latin]] word ''persona'' (person) means, while most Greek theologians would have interpreted that word to mean ''natura'' (nature). The energy and imprudence with which Eutyches asserted his opinions led to his being misunderstood. Thus, many believed that Eutyches was advocating [[Docetism]], a sort of reversal of [[Arianism]] – where Arius had denied the [[consubstantial]] divinity of [[Jesus]], Eutyches seemed to be denying that Jesus was fully human.<ref name=EB/> [[Pope Leo I]] wrote that Eutyches' error seemed to be more from a lack of skill than from malice.

Eutyches had been accusing various personages of covert Nestorianism. In November 448, [[Flavian of Constantinople|Flavian, Bishop of Constantinople]] held a local synod regarding a point of discipline connected with the province of [[Sardis]]. At the end of the session of this synod one of those inculpated, [[Eusebius of Dorylaeum|Eusebius, Bishop of Dorylaeum]], brought a counter charge of heresy against the [[archimandrite]].<ref>{{citation-attribution|1=[http://www.newadvent.org/cathen/05631a.htm Chapman, John. "Eutyches." The Catholic Encyclopedia] Vol. 5. New York: Robert Appleton Company, 1909. 6 February 2019}}</ref> Eusebius demanded that Eutyches be removed from office. Flavian preferred that the bishop and the archimandrite sort out their differences, but as his suggestion went unheeded, Eutyches was summoned to clarify his position regarding the nature of Christ. Eventually Eutyches reluctantly appeared, but his position was considered to be theologically unsophisticated, and the synod finding his answers unresponsive condemned and exiled him.<ref name=EB/> Flavian sent a full account to Pope Leo I. Although it had been accidentally delayed, Leo wrote a compendious explanation of the whole doctrine involved, and sent it to Flavian as a formal and authoritative decision of the question.<ref name=chapman>[http://www.newadvent.org/cathen/05019a.htm Chapman, John. "Dioscurus." The Catholic Encyclopedia] Vol. 5. New York: Robert Appleton Company, 1909. 8 February 2019</ref>

Eutyches appealed against the decision, labeling Flavian a Nestorian, and received the support of [[Pope Dioscorus I of Alexandria]]. [[John Anthony McGuckin]] sees an "innate rivalry" between the Sees of Alexandria and Constantinople.<ref>McGuckin, John Anthony. ''St. Cyril of Alexandria: The Christological Controversy''. Leiden: E.J. Brill, 1994. p. 12 {{ISBN|9789004099906}}</ref> Dioscurus, imitating his predecessors in assuming a primacy over Constantinople, held his own synod which annulled the sentence of Flavian, and absolved Eutyches.