A Fragment Preserved by Theodoret, from the Exposition of the Faith

We confess our Lord Jesus Christ, the only-begotten Son of God, begotten of the Father before all ages without beginning according to His divinity: but in the last days that the same was made incarnate of the holy Virgin Mary, and assumed a perfect man of a rational soul and body, consubstantial with the Father according to His divinity, and consubstantial with us according to His humanity. For the union of two perfect natures was made ineffably.

Therefore we confess one Christ, one Son, our Lord Jesus Christ, the Son of God, knowing that, being coeternal with His Father according to His divinity, according to which He is also the maker of all things, 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 thy word, to build for Himself ineffably a temple out of her, and to unite this to Himself from the very conception, not a coeternal body brought down from heaven out of His own substance, but assuming it from the mass of our substance, that is, out of the Virgin, and uniting it to Himself.

God the Word was not changed into flesh, nor did He appear as an apparition, but preserving His own substance without mutation or alteration, He united to Himself the assumed firstfruits of our nature. God the Word did not take His beginning from the Virgin, but being coeternal with His Father, He deigned through His immense goodness to unite to Himself the firstfruits of our nature, not commingled, but appearing one and the same in each substance, as it is written: Destroy this temple, and in three days I will raise it up. For Christ God is dissolved according to my substance, which He assumed: and He, the same, raises up the dissolved temple according to His divine substance, according to which He is also the maker of all things.

Never after the union, which He deigned to unite to Himself from the very conception, did He withdraw from His temple, nor, on account of His ineffable goodness, was He able to withdraw. But the same is both passible and impassible; passible according to His humanity, impassible according to His divinity. For, behold, behold Me, for I am, and I am not changed (Malachi 3:6). Therefore, His temple having been raised up, God the Word, with the resurrection and renewal of our nature perfected in Himself, showed this to His disciples, saying: Handle Me, and see, for a spirit hath not flesh and bones, as you see Me have; that considering Him who has and that which is had, you may understand not a mixture, not a conversion, nor an alteration, but a union that was made.

Therefore He also showed the marks of the nails and the wound of the spear, and ate before His disciples; that by every reasoning He might persuade them of the resurrection of our nature renewed in Himself. And while according to the blessed substance of His divinity He was without mutation and alteration, impassible, immortal, in need of nothing, He permitted all sufferings to be inflicted upon His temple, and by His own power He raised it up, and through His temple He effected the perfect renewal of our nature.

But those who say that Christ was a mere man, or that God the Word was subject to suffering, or that He was converted into flesh, or had a consubstantial body, or brought it down from heaven, or that it was a phantasm: or who say that God the Word, being mortal, had need to be raised up by the Father, or that He assumed a lifeless body, or a man without a mind, or that the two natures of Christ, through commingling, were confused into one nature: and who do not confess that in our Lord Jesus Christ there are two natures unconfused, but one person, inasmuch as Christ is one, the Son is one—these the catholic and apostolic Church anathematizes.