In 325, the first ecumenical council (First Council of Nicaea) established that Christ was God, "consubstantial" with the Father, against Arius's contention that he was a created being. This was reaffirmed at the First Council of Constantinople and the Council of Ephesus. In 449 the Second Council of Ephesus, which is not considered one of the ecumenical councils, implicitly upheld the teaching of Eutyches, that the Son had only one nature, divine and not human. The Council of Chalcedon convened to address this dispute. It set aside the findings of 449, and repudiated the idea that Jesus' mortal nature amounted to nothing. The Chalcedonian Creed describes the "full humanity and full divinity" of Jesus, the second person of the Holy Trinity.
The council issued 27 disciplinary canons governing church administration and authority. In the famous 28th canon passed by the council, the bishops sought to raise the See of Constantinople (New Rome) in stature, claiming that Constantinople enjoyed honor and authority similar to that of the See of (older) Rome. Pope Leo's legate opposed the canon and in 453, Pope Leo eventually confirmed all the canons, except for 28th.
The Council of Chalcedon is the fourth of the first seven Ecumenical Councils in Christianity, and is therefore recognized as infallible in its dogmatic definitions by the Roman Catholic and Eastern Orthodox Churches (then one church). The Trinity as defined by these councils is also taken as orthodox among most Protestants. However, the Council resulted in a major schism, with those who refused to accept its teaching, now known as Oriental Orthodoxy, being accused of monophysitism.
However, the works of two deceased Antiochean theologians, Diodorus of Tarsus and Theodore of Mopsuestia were at this time translated into Syriac. By the intervention of Archbishop Proclus of Constantinople, the two theologians were condemned throughout the East, but this situation would later provide the material for the Second Council of Constantinople some hundred years later.
Cyril had taught that "There is only one physis, since it is the Incarnation, of God the Word." Cyril had apparently understood the Greek word physis to mean approximately what the Latin word persona (person) means, while most Greek theologians would have interpreted that word to mean natura (nature). Thus, many understood Eutyches to be advocating Docetism, a sort of reversal of Arianism -- where Arius had denied the divine nature of Jesus, Eutyches seemed to be denying his human nature. (Cyril's orthodoxy was not called into question, since the Union of 433 had explicitly spoken of two physeis in this context.)
Pope Leo I, from Rome, wrote that Eutyches' error seemed to be more from a lack of skill on the matters than from malice. Further, his side of the controversy tended not to enter into arguments with their opponents, which prevented the misunderstanding from being uncovered. Nonetheless, due to the high regard in which Eutyches was held (second only to the Patriarch of Constantinople in the East), his teaching spread rapidly throughout the east.
In November 447, during a local synod in Constantinople, Eutyches was denounced as a heretic by the bishop of Dorylaeum, Eusebius, with the demand that he be removed from his office. The then archbishop, Flavian of Constantinople, did not wish to consider the matter as a result of the great prestige that Eutyches enjoyed, but finally relented, and so Eutyches was condemned as a heretic by the synod. However, the emperor Theodosius II and the Patriarch of Alexandria, Dioscorus, did not accept the decision of the synod because Eutyches had repented and confessed his orthodoxy. Dioscorus held his own synod reinstating Eutyches, and the emperor called a council to be held in Ephesus in 449, inviting Pope Leo I, who agreed to be represented by four legates (though one died en route).
The Second Council of Ephesus convened on August 8, 449, with some 130 bishops in attendance. Dioscorus presided by command of the emperor. The emperor denied the vote to any bishop who had voted in Eutyches' deposition two years earlier. As a result, there was a near-unanimous support for Eutyches, and Flavian was himself deposed and exiled. He died shortly thereafter. The papal legates left with a letter for the pope from Flavian, and in a second session, without papal representation, several more bishops were deposed, including Ibas of Edessa, Irenaeus of Tyre (a close personal friend of Nestorius), Domnus of Antioch, and Theodoret.
The decisions of this council threatened schism between the East and the West, since they went plainly against the papal declaration, although it was never read. The pope dubbed this council a "synod of robbers" — Latrocinium — and refused to accept its pronouncements. His letter was not read at the council and the papal legates left with it as well and it is for this reason that he called it so.
Marcian announced his intention to hold a new council, but not in Italy, as the pope had requested, but rather in the East. The council was called to meet at Nicaea, but was then moved at the last moment to Chalcedon, where the council opened on October 8, 451. Marcian had the exiled bishops returned to their dioceses and had the body of Flavian brought to the capital to be buried honorably.
The emperor asked the pope to preside over the council, but Pope Leo sent his legates instead--Bishops Pachasinus of Lilybaeum and Julian of Cos and two priests Boniface and Basil--to preside over the council in his name, condemn the work of the Latrocinium, and profess the correct doctrine about the Incarnation as described in his previous letter to Flavian (the Tome).
Attendance at this council was very high, from about 500 to 600 bishops. Paschasinus refused to give Dioscorus (who had carried out an excommunication of the pope in the period leading up to the council) a seat at the council, and as a result, he was moved to the nave of the church. Paschasinus further ordered the reinstatement of Theodoret and that he be given a seat, but this move caused such an uproar among the council fathers, that Theodoret also sat in the nave, though he was given a vote in the proceedings, which began with a trial of Dioscorus.
Marcian wished to bring proceedings to a speedy end, and asked the council to make a pronouncement on the doctrine of the Incarnation before continuing the trial. The council fathers, however, felt that no new creed was necessary, and that the doctrine had been laid out clearly in Leo's letter to Flavian, by then called the Tome The second day of the council ended with shouts from the bishops, "It is Peter who says this through Leo. This is what we all of us believe. This is the faith of the Apostles. Leo and Cyril teach the same thing."
The council continued with Dioscorus' trial, but he refused to appear before the assembly. As a result, he was condemned but by an underwhelming amount (more than half the bishops present for the previous sessions did not attend his condemnation), and all of his decrees were declared null. Marcian responded by exiling Dioscorus. All of the bishops were then asked to sign their assent to the Tome, but a group of thirteen Egyptians refused, saying that they would assent to "the traditional faith". As a result, the emperor's commissioners decided that a creed would indeed be necessary and presented a text to the fathers. No consensus was reached, and indeed the text has not survived to the present.
Paschasinus threatened to return to Rome to reassemble the council in Italy. Marcian agreed, saying that if a clause were not added to the creed supporting Leo's doctrine, the bishops would have to relocate. The bishops relented and added a clause, saying that, according to the decision of Leo, in Christ there are two natures united, inconvertible [natures], inseparable [natures].
The Confession of Chalcedon provides a clear statement on the human and divine nature of Christ:
Following the holy Fathers, we unanimously teach and confess one and the same Son, our Lord Jesus Christ: the same perfect in divinity and perfect in humanity, the same truly God and truly man, composed of rational soul and body; consubstantial with the Father as to his divinity and consubstantial with us as to his humanity; "like us in all things but sin." He was begotten from the Father before all ages as to his divinity and in these last days, for us and for our salvation, was born as to his humanity of the virgin Mary, the Mother of God.
We confess that one and the same Christ, Lord, and only-begotten Son, is to be acknowledged in two natures without confusion, change, division, or separation (in duabus naturis inconfuse, immutabiliter, indivise, inseparabilter). The distinction between natures was never abolished by their union, but rather the character proper to each of the two natures was preserved as they came together in one person (prosopon) and one hypostasis.
Interestingly enough, this goes against the teaching of Cyril from the previous council stating that it is incorrect to speak of Christ as existing in two natures after the union. The reasoning adopted by the Eastern Orthodox Church is that further clarification of Cyril's position was required.
The work of the council was completed by a series of 27 disciplinary canons:
Canon 28 grants equal privileges (isa presbeia) to Constantinople as of Rome because Constantinople is the New Rome as renewed by canon 36 of the Quinisext Council. The papal legates were not present for the vote on this canon, and protested it afterwards, and was not ratified by Pope Leo in Rome.
According to some ancient Greek collections, canons 29 and 30 are attributed to the council: canon 29, which states that an unworthy bishop cannot be demoted but can be removed, is an extract from the minutes of the 19th session; canon 30, which grants the Egyptians time to consider their rejection of Leo's Tome, is an extract from the minutes of the fourth session.
In all likelihood an official record of the proceedings was made either during the council itself or shortly afterwards. The assembled bishops informed the pope that a copy of all the "Acta" would be transmitted to him; in March, 453, Pope Leo commissioned Julian of Cos, then at Constantinople, to make a collection of all the Acts and translate them into Latin. Most of the documents, chiefly the minutes of the sessions, were written in Greek; others, e.g. the imperial letters, were issued in both languages; others, again, e.g. the papal letters, were written in Latin. Eventually nearly all of them were translated into both languages.
When the capital of the old Roman Empire was still in Rome, there was no pentarchy – that is, the five patriarchal sees whose bishops were the first among equals within their ecclesiastical provinces. Nonetheless, over time, bishops repeatedly asserted the authority of the bishop of Constantinople, although always second in honor to the bishop of Rome. For example, the third canon of the Second Ecumenical Council at Constantinople (381 AD) states that "the bishop of Constantinople... shall have the prerogative of honor after the bishop of Rome; because Constantinople is New Rome." It is possible that for the Eastern bishops Constantinople's preeminence was a fait accompli and that the 28th canon was a culmination of ongoing ecclesiological development. Or perhaps, as one commentator has noted, the claims of the Church in Rome regarding her preeminence "excited jealousy of her rival of the East.
In making their case, the council fathers argued that tradition had accorded "honor" to the see of older Rome because it was formerly the imperial city. Accordingly, “moved by the same purposes” the fathers “apportioned equal prerogatives to the most holy see of new Rome” because “the city which is honored by the imperial power and senate and enjoying privileges equaling older imperial Rome should also be elevated to her level in ecclesiastical affairs and take second place after her.” The framework for allocating ecclesiastical authority advocated by the council fathers mirrored the allocation of imperial authority in the later period of the Roman Empire. The Eastern position could be characterized as being pragmatic (or perhaps political) in nature, as opposed to a doctrinal view. In practice, all Christians East and West addressed the papacy as the See of Peter or the Apostolic See rather than the See of the Imperial Capital because it was commonly understood that Rome's precedence comes from Peter rather than its association with Imperial authority.
After the passage of the Canon 28, Rome filed a protest against the reduction of honor given to Antioch and Alexandria. However, growing concerns that withholding Rome's approval would be interpreted as a rejection of the entire council, in 453 he confirmed the council’s canons with a protest against the 28th.
Recent years have brought about a degree of rapprochement between Chalcedonian Christians and the Oriental Orthodox. Agreement on doctrine has been declared between Oriental Orthodox and Eastern Orthodox and Roman Catholic Churches, for instance, although communion between these families of churches has not been restored.