III. Eutyches' dogma is condemned by the testimony of Scripture and cannot further be entertained.
But nothing severs those who are deceived by their own inventions, from the light of the Gospel so much as their not thinking that the Lord's Incarnation appertains in a true sense to man's, that is, our, nature: as if it were unworthy of God's glory that the majesty of the impassible Word should have taken the reality of human flesh, whereas men's salvation could not otherwise have been restored had not He Who is in the form of God deigned also to take the form of a slave. And hence since the holy Synod of Chalcedon, which was attended by all the provinces of the Roman world and obtained universal acceptance for its decisions, and is in complete harmony therein with the most sacred council of Nicæa, has cut off all the wicked followers of the Eutychian dogma from the body of the Catholic communion, how shall any of the lapsed regain the peace of the church, without purging himself by a full course of penitence?
For what licence can be granted them for discussing, when they have deserved to be condemned by a just and holy judgment, so that they might most truly fall under that sentence of the blessed Apostle, wherewith at the very outset of the infant Church he overthrew the enemies of Christ's cross, saying: “every spirit which confesses Jesus Christ to have come in the flesh is of God, and every spirit which dissolves Jesus is not of God, but this is antichrist.” And this pre-existent teaching of the Holy Ghost we must faithfully and steadfastly make use of, lest, by admitting the discussions of such men the authority of the divinely inspired decrees be diminished, when in all parts of your kingdom and in all borders of the earth that Faith which was confirmed at Chalcedon is being established on the surest basis of peace, nor is any one worthy of the name of Christian who cuts himself off from communion with us.
Of whom the Apostle says, “a man that is heretical after a first and a second admonition, avoid, knowing that such a one is perverse and condemned by his own judgment.”
Source: Letters (New Advent)