We confess that our Lord Jesus Christ, the Only Begotten Son of God, was begotten before all ages, without beginning, of the Father, and that in these last days the same was made flesh of the holy Virgin Mary, assumed the manhood, in its perfection, of a reasonable soul and body, of one substance with the Father as touching His Godhead and of one substance with us as touching His manhood. For union of two perfect natures has been after an ineffable manner. Wherefore we acknowledge one Christ, one Son, our Lord Jesus Christ; knowing that being coeternal with His own Father as touching His Godhead, by virtue of which also He is creator of all, He deigned, after the assent of the Holy Virgin, when she said to the angel 'Behold the handmaid of the Lord, be it unto me according to your word' to build after an ineffable fashion a temple out of her for Himself, and to unite this temple to Himself by her conception, not taking and uniting with Himself a body coeternal with His own substance, and brought from heaven, but of the matter of our substance, that is of the Virgin. God the Word was not turned into flesh; His appearance was not unreal; keeping ever His own substance immutably and invariably He took the first fruits of our nature, and united them to Himself. God the Word did not take His beginning from the Virgin, but being coeternal with His own Father He of infinite kindness deigned to unite to Himself the first fruits of our nature, undergoing no mixture but in either substance appearing one and the same, as it is written 'Destroy this temple and in three days I will raise it up.' For the divine Christ, as touching my substance which he took is destroyed, and the same Christ raises the destroyed temple as touching the divine substance in which also He is Creator of all things. Never at any time after the Union which He deigned to make with Himself from the moment of the conception did He depart from His own temple, nor indeed through His ineffable love for mankind could depart.
The same Christ is both passible and impassible; as touching His manhood passible and as touching His Godhead impassible. 'Behold behold me, it is I, I have undergone no change'— and when God the Word had raised His own temple and in it had wrought out the resurrection and renewal of our nature, He showed this nature to His disciples and said 'Handle me and see for a spirit has not flesh and bones as you see me,' not 'be' but 'have.' So He says, referring to both the possessor and the possessed in order that you may perceive that what had taken place was not mixture, not change, not variation, but union. On this account too He showed the prints of the nails and the wound of the spear and ate before His disciples to convince them by every means that the resurrection of our nature had been renewed in Him; and further because in accordance with the blessed substance of His Godhead unchanged, impassible, immortal, He lived in need of nought, He by concession permitted all that can be felt to be brought to His own temple, and by His own power raised it up, and by means of His own temple made perfect the renewal of our nature.
“Them therefore that assert that the Christ was mere man, that God the Word was passible, or changed into flesh, or that the body which He had was consubstantial, or that He brought it from Heaven, or that it was an unreality; or assert that God the Word being mortal needed to receive His resurrection from the Father, or that the body which He assumed was without a soul, or manhood without a mind, or that the two natures of the Christ became one nature by confusion and commixture; them that deny that our Lord Jesus Christ was two natures unconfounded, but one person, as He is one Christ and one Son, all these the catholic and apostolic Church condemns.”
Of the same: —
“If then the flesh of all was in Christ or has been in Christ subject to wrongs, how can it be held to be of one essence with the Godhead? For if the Word and the flesh which derives its nature from earth are of one essence, then the Word and the soul which He took in its perfection are of one essence, for the Word is of one nature with God both according to the Word of the Father, and the confession of the Son Himself in the words, 'I and my Father are one.' Thus the Father must be held to be of the same substance with the body. Why any longer are you angry with the Arians, who say that the Son is a creature of God, while you assert yourselves that the Father is of one substance with His creatures?”
Of the same from his letter to the Emperor Gratianus: —
“Let us preserve a distinction between Godhead and flesh. One Son of God speaks in both, since in Him both natures exist. The same Christ speaks, yet not always in the same but sometimes in a different manner. Observe how at one time He expresses divine glory and at another human feeling. As God He utters the things of God, since He is the Word; as man He speaks with humility because He converses in my essence.”
On the same from the same book: —
“As to the passage where we read that the Lord of glory was crucified, let us not suppose that He was crucified in His own glory. But since He is both God and man, as touching His Godhead God, and as touching the assumption of the flesh, a man, Jesus Christ, the Lord of Glory, is said to have been crucified. For He partakes of either nature— that is the human and the divine. In the nature of manhood He underwent the passion in order that He who suffered might be said to be without distinction both Lord of Glory and Son of Man. As it is written 'He that came down from Heaven.'”
Similarly of the same: —
“Let then vain questions about words be silent, as it is written, the kingdom of God is not in 'enticing words' but in 'demonstration of the spirit.' For there is one Son of God who speaks in both ways, since both natures exist in Him; but although He Himself speaks He does not speak always in the same way; for you see in Him at one time God's glory, at another time man's feeling. As God He utters divine things, being the Word; as man He utters human things, since in this nature He spoke.”
Of the same from his work on the Incarnation of the Lord against the Apollinarians: —
“But while we are confuting these, another set spring up who assert the body of the Christ and His godhead to be of one nature. What hell has vomited forth so terrible a blasphemy? Really Arians are more tolerable, whose infidelity, on account of these men, is strengthened, so that with greater opposition they deny Father, Son and Holy Ghost to be of one substance, for they did at least endeavour to maintain the Godhead of the Lord and His flesh to be of one nature.”
Of the same (from the same chapter):—
“He has frequently told me that he maintains the exposition of the Nicene Council, but in that examination our Fathers laid down that the Word of God, not the flesh, was of one substance with the Father, and they confessed that the Word came from the substance of the Father but that the flesh is of the Virgin. Why then do they hold out to us the name of the Nicene Council, while in reality they are introducing innovations of which our forefathers never entertained the thought?”
Of the same against Apollinarius: —
“Refuse thou to allow that the body is by nature on a par with the Godhead. Even though thou believe the body of the Christ to be real and bring it to the altar for transformation, and fail to distinguish the nature of the body and of the Godhead we shall say to you, 'If you offer rightly and fail to distinguish rightly, you sin, hold your peace.' Distinguish what belongs naturally to us, and what is peculiar to the Word. For I had not what was naturally His, and He had not what was naturally mine, but He took what was naturally mine in order to make us partakers of what was His. And He received this not for confusion but for completion.”
Of the same, a little further on: —
Source: Dialogues ("Eranistes" or "Polymorphus") (New Advent)