Council of Chalcedon 451 Citation

" ..... So the proper character of both natures was maintained and came together in a single person. Lowliness was taken up by majesty, weakness by strength, mortality by eternity. To pay off the debt of our state, invulnerable nature was united to a nature that could suffer; so that in a way that corresponded to the remedies we needed, one and the same mediator between God and humanity the man Christ Jesus, could both on the one hand die and on the other be incapable of death. Thus was true God born in the undiminished and perfect nature of a true man, complete in what is his and complete in what is ours. By “ours” we mean what the Creator established in us from the beginning and what he took upon himself to restore. There was in the Saviour no trace of the things which the Deceiver brought upon us, and to which deceived humanity gave admittance. 

He took on the form of a servant (He controlled Human Being who was made) without the defilement of sin, thereby enhancing the human and not diminishing the divine. For that self-emptying whereby the Invisible rendered himself visible, and the Creator and Lord of all things chose to join the ranks of mortals, spelled no failure of power: it was an act of merciful favour. So the one who retained the form of God when he made humanity, was made man in the form of a servant. Each nature kept its proper character without loss; and just as the form of God does not take away the form of a servant, so the form of a servant does not detract from the form of God.

So without leaving his Father’s glory behind, the Son of God comes down from his heavenly throne and enters the depths of our world, born in an unprecedented order by an unprecedented kind of birth.

 We must say this again and again: one and the same is truly Son of God and truly son of man. God, by the fact that in the beginning was the Word, and the Word was with God, and the Word was God; man, by the fact that the Word was made flesh and dwelt among us. God, by the fact that all things were made through him, and nothing was made without him. man, by the fact that he was made of a woman, made under the law. 

.... All this was so that it would be recognised that the proper character of the divine and of the human nature went on existing inseparable in him; and so that we would realise that the Word is not the same thing as the flesh, but in such a way that we would confess belief in the one Son of God as being both Word and flesh. Both Word and Flesh are United, not Separated, but not alterable.

This Eutyches must be judged to be extremely destitute of this mystery of the faith. Neither the humility of the mortal life nor the glory of the resurrection has made him recognise our nature in the only-begotten of God. Nor has even the statement of the blessed apostle and evangelist John put fear into him: Every spirit which confesses that Jesus Christ came in the flesh is from God, and every spirit which puts Jesus asunder is not from God, and this is Antichrist. But what does putting Jesus as under consist in if not in separating his human nature from him, and in voiding, through the most barefaced fictions, the one mystery by which we have been saved? Once in the dark about the nature of Christ’s body, it follows that the same blindness leads him into raving folly about his suffering too. If he does not think that the Lord’s cross was unreal and if he has no doubt that the suffering undergone for the world’s salvation was real, then let him acknowledge the flesh of the one whose death he believes in. And let him not deny that a man whom he knows to have been subject to suffering had our kind of body, for to deny the reality of the flesh is also to deny the bodily suffering. So if he accepts the christian faith and does not turn a deaf ear to the preaching of the gospel, let him consider what nature it was that hung, pierced with nails, on the wood of the cross. With the side of the crucified one laid open by the soldier’s spear, let him identify the source from which blood and water flowed, to bathe the church of God with both font and cup." 

(Council of Chalcedon 451: The letter of Pope Leo on to Flavian, bishop of Constantinople, about Eutyches on His Monophysitism Heresy)