III. The soul of Christ and the body of Christ were real in the full human sense, though the circumstances of His birth were unique.
[But as to that which Eutyches dared to say in the court of bishops “that before the Incarnation there were two natures in Christ, but after the Incarnation one,” he ought to have been pressed by the frequent and anxious questions of the judges to render an account of his acknowledgment, lest it should be passed over as something trivial, though it was seen to have issued from the same fount as his other poisonous opinions. For I think that in saying this he was convinced that the soul, which the Saviour assumed, had had its abode in the heavens before He was born of the Virgin Mary, and that the Word joined it to Himself in the womb.
But this is intolerable to Catholic minds and ears: because the Lord who came down from heaven brought with Him nothing that belonged to our state: for He did not receive either a soul which had existed before nor a flesh which was not of his mother's body. Undoubtedly our nature was not assumed in such a way that it was created first and then assumed, but it was created by the very assumption. And hence that which was deservedly condemned in Origen must be punished in Eutyches also, unless he prefers to give up his opinion, viz. the assertion that souls have had not only a life but also different actions before they were inserted in men's bodies].
For although the Lord's nativity according to the flesh has certain characteristics wherein it transcends the ordinary beginnings of man's being, both because He alone was conceived and born without concupiscence of a pure Virgin, and because He was so brought forth of His mother's womb that her fecundity bare Him without loss of virginity: yet His flesh was not of another nature to ours: nor was the soul breathed into Him from another source to that of all other men, and it excelled others not in difference of kind but in superiority of power.
For He had no opposition in His flesh [nor did the strife of desires give rise to a conflict of wishes]. His bodily senses were active without the law of sin, and the reality of His emotions being under the control of His Godhead and His mind, was neither assaulted by temptations nor yielded to injurious influences. But true Man was united to God and was not brought down from heaven as regards a pre-existing soul, nor created out of nothing as regards the flesh: it wore the same person in the Godhead of the Word and possessed a nature in common with us in its body and soul.
For He would not be “the mediator between God and man,” unless God and man had co-existed in both natures forming one true Person. The magnitude of the subject urges us to a lengthy discussion: but with one of your learning there is no need for such copious dissertations, especially as we have already sent a sufficient letter to our brother Flavian by our delegates for the confirmation of the minds, not only of priests but also of the laity. The mercy of God will, we believe, provide that without the loss of one soul the sound may be defended against the devil's wiles, and the wounded healed. Dated 13th June in the consulship of the illustrious Asturius and Protogenes (449).
Source: Letters (New Advent)