The Chalcedonian schism, also known as the Monophysite schism, is the break of communion between the Oriental Orthodox Churches and the Chalcedonian Christianity (which later gave both the Eastern Orthodox Church and the Catholic Church) in the aftermath of the Council of Chalcedon. Although the bishops at Chalcedon greatly respected Cyril of Alexandria and used his writings as a benchmark for orthodoxy, opponents of the council believed that the Chalcedonian Definition, which states that Christ is "acknowledged in Two Natures", was too close to Nestorianism and contradicts Cyril's formula "one nature of God the Word incarnate". The Council had also deposed the Pope of Alexandria, Dioscorus, but his supporters continued to consider him their rightful Pope, refusing to recognise the council-appointed Proterius.
The anti-Chalcedonian strongholds were in Egypt, Palestine and later Syria. Over the next century, their communities gradually separated from the official church of the Byzantine Empire, eventually becoming the Oriental Orthodox Churches. The imperial government made many attempts to mend the schism, generally by trying to compromise between the two positions, but these attempts only created further heresies and schisms. The Arab conquests of the Levant and of Egypt in the 7th century fossilised the schism, but ecumenical dialogue between Chalcedonian and non-Chalcedonian Christians has been renewed since the 20th century.
Background
Council of Ephesus
Leading up to the 430s, the theological Schools of Alexandria and of Antioch had been developing slightly different views on Christology. Antiochene theologians such as Theodore of Mopsuestia and Theodoret of Cyrrhus had a "Word-man" Christology, or "Christology from below", which placed more emphasis on Christ's humanity and free will, and the distinction of his two natures. Their view was that Jesus, the man (homo assumptus) to whom God the Word was conjoined, atoned for the sins of mankind through his perfect life and his sacrifice on the cross, after which God raised him up to heaven "as a pledge of the salvation of all humanity". In contrast, Alexandrian theologians such as Athanasius and Cyril of Alexandria had a "Word-flesh" Christology, or "Christology from above", which placed more emphasis on Christ's divinity and its absolute unity with his humanity. In their view, the Word became flesh, not merely by indwelling a man but through a hypostatic union. He then "tasted death in the flesh" on the cross and rose from the dead, thus saving humanity not by a moral example but by being incarnate for our sake, since "what is not assumed is not saved".
Additionally, the See of Rome's interpretation of its relationship to the other patriarchates (Constantinople, Alexandria, and Antioch) was increasingly diverging from their own. While the other patriarchates saw themselves as colleagues based on their autocephalous relationship to each other, the Popes of Rome believed that their position as "first among equals" and as the successors of Saint Peter gave them authority over the entire church.
These divisions came to a head in April 428, when Theodosius II appointed Nestorius, a monk from Antioch, to be the Patriarch of Constantinople. Nestorius immediately outraged the population by preaching against the use of the title Theotokos (Mother of God) for the Virgin Mary, arguing that Mary only gave birth to Christ's humanity, not his divinity. This was despite the title having a large role in popular piety, and having been used by previous theologians such as Athanasius, the Cappadocian Fathers, John Chrysostom, and even (with reservations) by Nestorius' own teacher Theodore of Mopsuestia. Nestorius' main opponent was Cyril, the Patriarch of Alexandria, who wrote him three letters. In the second letter, written in February 430, Cyril criticised Nestorius' theology, arguing that the Word was not united to a man "merely as a result of will or good pleasure", but "became flesh" as is written in John 1:14, and therefore the Virgin who bore the Word in his human nature can rightly be called the Theotokos. Cyril was more aggressive in his third letter to Nestorius, which further defined his theology and ended with 12 prepositions Cyril called on Nestorius to anathematize.
Instead of cornering Nestorius, Cyril's third letter brought his own orthodoxy into question. Theodoret of Cyrrhus and Andrew of Samosata wrote rebuttals of Cyril's 12 anathemas, to which he responded with counter-rebuttals. Both Cyril and Nestorius wrote to Pope Celestine I of Rome for support, but he sided with Cyril. To settle the dispute, Theodosius II convened the Council of Ephesus. On 22 June 431, the council condemned Nestorius as "the new Judas" and deposed him. However, the dispute was not over, as John of Antioch arrived 4 days later with a group of Syrian bishops, only to find the council already finished and Nestorius condemned. He then excommunicated Cyril and everyone who had agreed to his 12 Anathemas. In April 433, Cyril and John reconciled with one another by both agreeing to the Formula of Reunion, which stated that Christ was "of two natures" (Greek: Δύο φύσεων ἕνωσις), in a "union without confusion" (Greek: ἀσυγχύτου ἑνώσεως), and consubstantial both with the Father and with humanity.
Some of Cyril's supporters believed that the Formula of Reunion was not compatible with the 12 Anathemas, and that by accepting the former he had renounced the latter. Cyril sent many letters to these people, explaining that the Formula of Reunion was compatible with the 12 Anathemas and that he still stood by both. In some of these letters, like his second letter to Succensus, Bishop of Diocaesarea, he argued that Christ could be spoken of as having two natures after his Incarnation as long as the two natures are divided "in thought only" (Greek: τῇ θεωρίᾳ μόνῃ), the way a human body and soul can be considered different but are in fact just one person, with no real, observable division.
Second Council of Ephesus
After the Council of Ephesus, there was peace for about a decade, but theological controversies soon brewed up again in the 440s. In 447, an archimandrite in Constantinople named Eutyches, apparently misunderstanding Cyril's "one nature of God the Word incarnate" formula, began preaching that Christ has only one nature, and that his humanity was so completely "deified" by his divinity that he was not consubstantial with mankind. Bishop Eusebius of Dorylaeum, who had been one of the first people to denounce Nestorius, presented a bill of indictment formally accusing Eutyches of heresy. In November 448, Eutyches was tried by the Home Synod of Constantinople, presided by the Patriarch Flavian of Constantinople. After he refused to confess two natures after the Incarnation, the synod excommunicated Eutyches, judging that he had overreacted to Nestorianism and fallen into the opposite heresy.
Flavian sent a report of Eutyches' trial to Pope Leo I, who responded with a statement of faith known as Leo's Tome. Meanwhile, Eutyches did not accept his condemnation quietly. He appealed to Emperor Theodosius II, as well as to Pope Dioscorus I of Alexandria, who had succeeded Cyril. Through the influence of Chrysaphius, Eutyches' godson and a powerful eunuch in the imperial court, Theodosius convoked another council at Ephesus to re-try Eutyches, this time presided by Dioscorus. This Second Council of Ephesus began on 8 August 449, and was attended by Flavian, Juvenal of Jerusalem, Domnus of Antioch, and about 130 other bishops. During this council, the Roman papal legates repeatedly called for Leo's Tome to be read, but Dioscorus intentionally ignored them. After having the acts of the 448 Home Synod read, Dioscorus declared that Flavian and Eusebius were guilty of innovation, and pronounced them deposed. The council immediately broke into chaos, with armed monks and soldiers charging into the church. Flavian was beaten so badly he died of his injuries; according to eyewitness Diogenes of Cyzicus, an archimandrite named Barsauma stood by and cried "Strike him dead!" as his monks beat Flavian. Although the acts of the council recorded that the bishops present assented to Eusebius and Flavian's depositions, two years later they would claim that Dioscorus used armed monks and soldiers to threaten them, had them sign blank papers, and forged statements from them.
Despite Dioscorus' attempts to stop him, the papal legate Hilarius escaped Ephesus with great difficulty and made it back to Rome, bringing with him a letter Flavian had written to Leo shortly before his death. Leo was outraged by the Second Council of Ephesus, dubbing it a latrocinium ("synod of robbers") in a letter to Pulcheria. He wrote many appeals to the emperor, the court, the clergy and the people of Constantinople urging them not to accept it, but Theodosius refused to reconsider its decrees.
Council of Chalcedon
In July 450, Theodosius II died in a hunting accident and was succeeded by his sister Pulcheria. She immediately had Chrysaphius executed, and married the general Marcian on the condition that he allow her to keep her vow of chastity. She also reached out to Leo, who demanded the acceptance of his Tome as a condition for re-establishing communion. Anatolius and Maximus (who had been installed by Dioscorus as the patriarchs of Constantinople and Antioch respectively) assented, and Ephesus II rapidly lost support. The main criticism against it was not its theology, but the brutal methods with which it was conducted. In 451, the new emperor Marcian summoned another council in Chalcedon, "to end the disputations and settle the true faith more clearly and for all time."
The Council of Chalcedon was attended by 520 bishops, of whom 124 had been present at Ephesus II. In its second session (held on 10 October), they read Leo's Tome and the letters of Cyril, and, after vigorous debate, they declared them to be in agreement with one another, thus accepting Leo's Tome as orthodox. Cyril's third letter to Nestorius, which contained his 12 Anathemas and his most strongly miaphysite statements, was notably absent from the discussion. The council's third session on 13 October was a trial of Dioscorus. Several accusers, most of whom had been Cyril's family or friends, listed their grievances against Dioscorus' conduct. He was repeatedly summoned to defend himself, but claimed he was under house arrest and could not come to the council, so he was deposed in absentia. Importantly, this deposition was not for heresy, but for "grave administrative errors" like allowing Flavian's death.
In the fifth session, the imperial officials asked the bishops to compose a Definition of Faith, which would become the Chalcedonian Definition. The bishops were reluctant to do so, since Canon 7 of Ephesus forbade the use of new creeds, but eventually they wrote a draft that contained the Cyrillian phrase "of two natures" (Greek: ἐκ δύο φύσεων). Although the vast majority of the council was pleased with this draft, the papal legates protested and threatened to go home. The bishops then wrote another version, using "in two natures" (Greek: ἐν δύο φύσεσιν) instead. Many of the bishops were uncomfortable with this clause; only two years before, at Ephesus II, they had shouted "Anathema to whoever says two natures after the incarnation", and called for those who divide Christ in two to be cut into pieces. These bishops acquiesced only after the imperial officials reminded them that they had already accepted the orthodoxy of Leo's Tome, which said that "there are two natures in Christ, united without confusion, change or separation in the one only-begotten Son our Saviour".
The Chalcedonian Definition was largely based on the 433 Formula of Reunion, with some additions from Cyril's other letters. It attempted to define the via media between the opposite heresies of Eutyches and Nestorius. For example, it used the phrase "unconfusedly, unchangeably, indivisibly, inseparably" (Greek: ἀσυγχύτως, ἀτρέπτως, ἀδιαιρέτως, ἀχωρίστως); the first two words were directed at Eutyches, and the second two at Nestorius.
Aftermath
The Council of Chalcedon faced immediate opposition in the eastern parts of the Empire. Syria was divided between pro- and anti-Chalcedonians, while in Egypt the vast majority were against Chalcedon. They felt that Leo's Tome and the Chalcedonian Definition, despite condemning Nestorius in name, had restored his heretical teachings by describing Christ as "in two natures". Rumours about Chalcedon spread in Egypt, claiming that it had vindicated Nestorius' teachings, that it taught that the one crucified was not God, that it was not scriptural, and that the Jews were delighted to hear of it. Leo complained that Greek translations of his Tome misleadingly made him speak of "two persons". These rumours were not helped by the fact that Chalcedon had restored two Nestorian bishops to their sees, Theodoret of Cyrrhus and Ibas of Edessa, although that was on the condition that they anathematise Nestorius, which they did. The opposition to Chalcedon initially included respectable monastic figures like Saint Gerasimus, Saint Gerontius (former confidant of Melania the Younger), Peter the Iberian, and the former empress Aelia Eudocia.