V. Nestorianism and Eutychianism are particularly to be avoided at the present time
There are many other astounding falsehoods also which we must not weary your ears, beloved, with enumerating. But after all these various impieties, which are closely connected by the relationship that exists between one form of blasphemy and another, we call your devout attention to the avoiding of these two errors in particular: one of which, with Nestorius for its author, some time ago attempted to gain ground, but ineffectually; the other, which is equally damnable, has more recently sprung up with Eutyches as its propounder.
The former dared to maintain that the blessed Virgin Mary was the mother of Christ's manhood only, so that in her conception and childbearing no union might be believed to have taken place of the Word and the Flesh: because the Son of God did not Himself become Son of Man, but of His mere condescension linked Himself with created man. This can in no way be tolerated by catholic ears, which are so imbued with the gospel of Truth that they know of a surety there is no hope of salvation for mankind unless He were Himself the Son of the Virgin who was His mother's Creator.
On the other hand this blasphemous propounder of more recent profanity has confessed the union of the two Natures in Christ, but has maintained that the effect of this very union is that of the two one remained while the substance of the other no longer existed, which of course could not have been brought to an end except by either destruction or separation. But this is so opposed to sound faith that it cannot be entertained without loss of one's Christian name. For if the Incarnation of the Word is the uniting of the Divine and human natures, but by the very fact of their coming together that which was twofold became single, it was only the Godhead that was born of the Virgin's womb, and went through the deceptive appearance of receiving nourishment and bodily growth: and to pass over all the changes of the human state, it was only the Godhead that was crucified, dead, and buried: so that according to those who thus think, there is no reason to hope for the resurrection, and Christ is not “the first-begotten from the dead;” because He was not One who ought to have been raised again, if He had not been One who could be slain.
Source: Sermons (New Advent)