From St John Damascene's Exact Exposition of the Orthodox Faith. This is meant to show that the evidence that an Orthodox Christian must believe in two natures, not one, is not just found in the Fourth Ecumenical Synod:
Chapter III.—Concerning Christ’s two natures, in opposition to those who hold that He has only one
For the two natures were united with each other without change or alteration, neither the divine nature departing from its native simplicity, nor yet the human being either changed into the nature of God or reduced to non-existence, nor one compound nature being produced out of the two. For the compound nature cannot be of the same essence as either of the natures out of which it is compounded, as made one thing out of others: for example, the body is composed of the four elements, but is not of the same essence as fire or air, or water or earth, nor does it keep these names. If, therefore, after the union, Christ’s nature was, as the heretics hold, a compound unity, He had changed from a simple into a compound nature, and is not of the same essence as the Father Whose nature is simple, nor as the mother, who is not a compound of divinity and humanity. Nor will He then be in divinity and humanity: nor will He be called either God or Man, but simply Christ: and the word Christ will be the name not of the subsistence, but of what in their view is the one nature.
We, however, do not give it as our view that Christ’s nature is compound, nor yet that He is one thing made of other things and differing from them as man is made of soul and body, or as the body is made of the four elements, but hold that, though He is constituted of these different parts He is yet the same. For we confess that He alike in His divinity and in His humanity both is and is said to be perfect God, the same Being, and that He consists of two natures, and exists in two natures. Further, by the word “Christ” we understand the name of the subsistence, not in the sense of one kind, but as signifying the existence of two natures. For in His own person He anointed Himself; as God anointing His body with His own divinity, and as Man being anointed. For He is Himself both God and Man. And the anointing is the divinity of His humanity. For if Christ, being of one compound nature, is of like essence to the Father, then the Father also must be compound and of like essence with the flesh, which is absurd and extremely blasphemous.
How, indeed, could one and the same nature come to embrace opposing and essential differences? For how is it possible that the same nature should be at once created and uncreated, mortal and immortal, circumscribed and uncircumscribed?
But if those who declare that Christ has only one nature should say also that that nature is a simple one, they must admit either that He is God pure and simple, and thus reduce the incarnation to a mere pretence, or that He is only man, according to Nestorius. And how then about His being “perfect in divinity and perfect in humanity”? And when can Christ be said to be of two natures, if they hold that He is of one composite nature after the union? For it is surely clear to every one that before the union Christ’s nature was one.
But this is what leads the heretics, that they look upon nature and subsistence as the same thing. For when we speak of the nature of men as one, observe that in saying this we are not looking to the question of soul and body. For when we compare together the soul and the body it cannot be said that they are of one nature. But since there are very many subsistences of men, and yet all have the same kind of nature: for all are composed of soul and body, and all have part in the nature of the soul, and possess the essence of the body, and the common form: we speak of the one nature of these very many and different subsistences; while each subsistence, to wit, has two natures, and fulfils itself in two natures, namely, soul and body.
But a common form cannot be admitted in the case of our Lord Jesus Christ. For neither was there ever, nor is there, nor will there ever be another Christ constituted of deity and humanity, and existing in deity and humanity at once perfect God and perfect man. And thus in the case of our Lord Jesus Christ we cannot speak of one nature made up of divinity and humanity, as we do in the case of the individual made up of soul and body. For in the latter case we have to do with an individual, but Christ is not an individual. For there is no predicable form of Christlihood, so to speak, that He possesses. And therefore we hold that there has been a union of two perfect natures, one divine and one human; not with disorder or confusion, or intermixture, or commingling, as is said by the God-accursed Dioscorus and by Eutyches and Severus, and all that impious company: and not in a personal or relative manner, or as a matter of dignity or agreement in will, or equality in honour, or identity in name, or good pleasure, as Nestorius, hated of God, said, and Diodorus and Theodorus of Mopsuestia, and their diabolical tribe: but by synthesis; that is, in subsistence, without change or confusion or alteration or difference or separation, and we confess that in two perfect natures there is but one subsistence of the Son of God incarnate; holding that there is one and the same subsistence belonging to His divinity and His humanity, and granting that the two natures are preserved in Him after the union, but we do not hold that each is separate and by itself, but that they are united to each other in one compound subsistence. For we look upon the union as essential, that is, as true and not imaginary. We say that it is essential, moreover, not in the sense of two natures resulting in one compound nature, but in the sense of a true union of them in one compound subsistence of the Son of God, and we hold that their essential difference is preserved. For the created remaineth created, and the uncreated, uncreated: the mortal remaineth mortal; the immortal, immortal: the circumscribed, circumscribed: the uncircumscribed, uncircumscribed: the visible, visible: the invisible, invisible. “The one part is all glorious with wonders: while the other is the victim of insults.”
Moreover, the Word appropriates to Himself the attributes of humanity: for all that pertains to His holy flesh is His: and He imparts to the flesh His own attributes by way of communication in virtue of the interpenetration of the parts one with another, and the oneness according to subsistence, and inasmuch as He Who lived and acted both as God and as man, taking to Himself either form and holding intercourse with the other form, was one and the same. Hence it is that the Lord of Glory is said to have been crucified, although His divine nature never endured the Cross, and that the Son of Man is allowed to have been in heaven before the Passion, as the Lord Himself said. For the Lord of Glory is one and the same with Him Who is in nature and in truth the Son of Man, that is, Who became man, and both His wonders and His sufferings are known to us, although His wonders were worked in His divine capacity, and His sufferings endured as man. For we know that, just as is His one subsistence, so is the essential difference of the nature preserved. For how could difference be preserved if the very things that differ from one another are not preserved? For difference is the difference between things that differ. In so far as Christ’s natures differ from one another, that is, in the matter of essence, we hold that Christ unites in Himself two extremes: in respect of His divinity He is connected with the Father and the Spirit, while in respect of His humanity He is connected with His mother and all mankind. And in so far as His natures are united, we hold that He differs from the Father and the Spirit on the one hand, and from the mother and the rest of mankind on the other. For the natures are united in His subsistence, having one compound subsistence, in which He differs from the Father and the Spirit, and also from the mother and us.