II. And in particular we must abstain from heresy, and that of Eutyches as well as that of Nestorius
We must restrain ourselves, therefore, from food, but much more must we fast from errors that the mind, given up to no carnal pleasure, may be taken captive by no falsehood: because as in past days, so also in our own, there are not wanting enemies of the Truth, who dare to stir up civil wars within the catholic Church, in order that by leading the ignorant into agreement with their ungodly doctrines they may boast of increase in numbers through those whom they have been able to sever from the Body of Christ.
For what is so opposed to the Prophets, so repugnant to the Gospels, so at variance with the Apostles' teaching as to preach one single Nature in the Lord Jesus Christ born of Mary, and without respect to time co-eternal with the Eternal Father? If it is only man's nature which is to be acknowledged, where is the Godhead Which saves? If only God's, where is the humanity which is saved? But the catholic Faith, which withstands all errors, refutes these blasphemies also at the same time, condemning Nestorius, who divides the Divine from the human, and denouncing Eutyches, who nullifies the human in the Divine; seeing that the Son of True God, Himself True God, possessing unity and equality with the Father and with the Holy Ghost, has vouchsafed likewise to be true Man, and after the Virgin Mother's conception was not separated from her flesh and child-bearing, so uniting humanity to Himself as to remain immutably God; so imparting Godhead to man as not to destroy but enhance him by glorification.
For He, Who became “the form of a slave,” ceased not to be “the form of God,” and He is not one joined with the other, but One in Both, so that ever since “the Word became Flesh” our faith is disturbed by no vicissitudes of circumstance, but whether in the miracles of power, or in the degradation of suffering, we believe Him to be both God, Who is Man, and Man, Who is God.
Source: Sermons (New Advent)