The Fourth Ecumenical Council, at which 630 bishops participated, was convened in the year 451 in the city of Chalcedon under the emperor Marcian (450-457). Still back in the time of the emperor Theodosius II (408-450), the bishop of Dorileuseia Eusebios in 408 reported to a Council held at Constantinople under the holy Patriarch Flavian (Comm. 18 February), concerning a personage of one of the monasteries of the capital, the archimandrite Eutykhios, who in his undaunted zeal against the soul-destroying heresy of the Nestorius -- went to the opposite extreme and began to assert, that within Jesus Christ the human nature under the hypostatic union was completely absorbed by the Divine nature, in consequence of which it lost everything characteristic of human nature, except but for the visible form; wherein, such that after the union in Jesus Christ there remained only one nature (the Divine), which in visible bodily form lived upon the earth, suffered, died, and was resurrected.
The Constantinople Council condemned this new false-teaching. But the heretic Eutykhios had patronage at court, and was in close connection with the heretic Dioskoros, the successor to Sainted Cyril (Comm. 18 January) upon the patriarchal see at Alexandria. Eutykhios turned to the emperor with a complaint against the injustice of the condemnation against him, and he demanded the judgement of an Ecumenical Council against his opponents, whom he accused of Nestorianism. Wanting to restore peace in the Church, Theodosius had decided to convene a Fourth Ecumenical Council in the year 449 at Ephesus. But this Council became branded in the chronicles of the Church as the "Robbers Council." Dioskoros, appointed by the emperor to preside as president of the Council, ran it like a dictator, making use of threats and outright coercion. Eutykhios was exonerated, and Saint Flavian condemned. But in the year 450 the emperor Theodosius died. The new emperor Marcian raised up onto the throne with him the sister of Theodosius, Pulcheria.
Restoring peace to the Church was a matter of prime importance. An Ecumenical Council was convened in the year 451 at Chalcedon. The Patriarch of Constantinople, Saint Anatolios (Comm. 3 July) presided over the Council. Dioskoros at the first session was deprived of his place among those present, and at the third session he was condemned with all his partisans. The Sessions of the Council were 16 in all. The Chalcedon holy fathers pronounced anathemas against the heresy of Eutykhios. On the basis of Letters Saint Cyril of Alexandria and Pope Saint Leo the Great, the fathers of the Council resolved:
"Following the holy fathers, we all with one accord teach to confess as one and the same the Son, our Lord Jesus Christ, perfect in Divinity and perfect in humanity, truly God, truly man, of Whom is a reasoned soul and a body, One in Essence with the Father through Divinity and that Same-One one-in-essence with us through humanity, in all things like unto us except for sin, begotten before the ages from the Father in Divinity, but in these latter days born for us and our salvation from Mary the Virgin Mother of God in humanity. This self-same Christ, Son and Lord, the Only-Begotten, is in two natures perceived without mingling, without change, without division, without separation [Greek: "asugkhutos, atreptos, adiairetos, akhoristos"; Slavic: "neslitno, neizmenno, nerazdel'no, nerazluchno"], such that by conjoining there be not infringement of the distinctions of the two natures, and by which is preserved the uniqueness of each nature conjoined in one Person and One Hypostasis, -- not split nor separated into two persons, but rather the One and Self-same Son, the Only-Begotten, the Word of God, the Lord Jesus Christ, as in antiquity the prophets taught of Him and as the Lord Jesus Christ Himself taught us, and as the Creed-Symbol of the fathers has passed down to us."
In the two final Sessions of the Council, 30 Canon-rules were promulgated concerning ecclesial hierarchies and disciplines. Beyond this, the Council affirmed the decrees not only of the three preceding Ecumenical Councils, but also of the Local Councils of: Ancyra, Neocaesarea, Gangra, Antioch and Laodiceia, which had occurred during the fourth century.