You may not have directly referenced it, but the question of ontological relationship you based your argument on requires a Platonic understanding of ontology to some degree. Natures have to exist in a real sense as some kind of universal, unlike in nominalism where the idea of a "human nature" is simply a convenient way to categorize similar expressions of individuals. Western philosophy has leaned so heavily into nominalism that the error of Apollinarianism doesn't make sense to someone who doesn't believe universals exist in any real sense.
Once again, you accuse me of a dependency on Platonism that I don’t have, and indeed, purposefully avoided. I never once spoke of the human nature or the divine nature of Christ, and the reason for this is because the Oriental Orthodox are miaphysites, believing our Lord exists from two natures, human and divine, whereas Chalcedonians believe our Lord exists in two natures, human and divine.
What unites both, and also the Church of the East, since it adopted a Syriac psuedo-Chalcedonian system based on qnume instead of nature or hypostasis, a different nominal construct altogether, is that all of the above believe Christ our True God is fully Man and fully God, without change, confusion, separation or division.
It is possible to differentiate between mankind and God, and to identify humanity without resort to Platonic ontology: homo sapiens as a distinct biological species identified through our DNA, which is different from that of all other animals. According to Scripture, God created us; He created us male and female, and He created us in His image, and when we fell into Sin he caused us to be mortal, limiting our lifespan according to Orthodox theology as a mercy, since indefinite existence in this fallen world would be intolerable. There exists a scriptural, Biblical definition of humanity which in no respect requires use of Plato.
God, on the other hand, is also scripturally defined: He is infinite, His name means “I am that I will be”, He is unchanging, He abides in the Father, who no man has seen at any time, the only begotten Son and Word of God, our savior, Jesus Christ, who is God with Us (emanu-el), in whom fullness of the invisible God dwelled bodily, and through whom we can see the Father, and finally the Holy Spirit, who has been made our Comforter and Paraclete, but in contrast to the Father, who no one has seen at any time, the Holy Spirit has only ever been seen, and also experienced otherwise, for in baptism we receive the life-giving Spirit. The early church fathers used the Greek word prosopon to refer to the Father, Son and Holy Ghost, which is a good choice of a word since it literally translates into Latin as person, but originally in Greek referred to mask, and had the implication of persona, personality or visage; slaves in the ancient Roman Empire had no prosopon - their personhood was denied, whereas Christianity requires us to treat our neighbor as we would be treated, so we have no right to deny anyone personhood.
The Gospel of John makes it clear that Jesus Christ is fully God, as defined scripturally, without recourse to a Platonic ideal of God, and fully Man, as defined scripturally, without reference to a Platonic ideal, although in a sense St. Paul and the other Apostles made use of the concept in describing our Lord as “the New Adam” in inspired writings, and St. John does refer to Christ as the Logos, and his divinely inspired use of the word Logos has much in common with the Platonic idea of the Logos, but it has more in common with the Memre, and it specifically derives from the claim of Christ to be the Truth, the Way and the Light.
This is also what the Nicene Creed / CF Statement of Faith says. The Nicene Creed makes use of the Hellenic word “ousia” to say that our Lord is of one essence with the Father, but this is Aristotelian and not Platonic.
If Nominalism is taken to an extreme where you accept Apollinarianism, which requires rejecting the idea that a human must have a human soul, and also requires saying that God has a soul in the same way we do, which is also a problematic statement since Scripture makes it clear that God is inscrutable, and the word soul in its ancient context is defined to specific concepts, and so the idea of God having a soul which could inhabit a human body requires us to say a lot of things about God that are not contained in Scripture, including denying His infinity, then Nominalism starts to become a problem.
However I myself approached this issue not using Platonic philosophy any more than the Nicene Fathers, but using a Nominalist approach, because the differences between Chalcedonian Christianity, Oriental Orthodoxy and the Assyrian Church of the East are
nominal, since all three of them define Christ as being fully human and fully divine, without change, confusion, separation or division.
Apollinarianism is one of a number of theologies that confuses the humanity and divinity, causing Christ to cease to be fully God and fully Man, preventing Him from being the new Adam, which is why it was anathematized directly in the new Creed, which declares that Jesus Christ is begotten, not made, begotten of the Father before all worlds, of one essence with the Father, very God of very God, who for us men and our salvation became incarnate of the Holy Spirit and the Virgin Mary, and became man.
The Christian Forums Statement of Faith, which includes the Nicene Creed, also summarizes this brilliantly:
“Faith groups and individuals that deny the full, eternal deity of Jesus Christ or His incarnation whereby He, as God, took on human flesh (becoming fully God and fully man in one person), are considered non-Christians at CF.”
Thus Nestorius and Apollinarius would both be considered non-Christians, because Nestorius denied that Christ was fully God and fully man in one person, and Apolliinarius likewise, but where Nestorius advocated a Christology where the human Christ was one person and the Divine logos another, united by a single will (requiring, ironically, Monothelitism)*, Apollinarius reduced the humanity of Christ to having a human body, ignoring the importance of the soul in human beings, which is doctrinally important in all Christiaan denominations, to the extent that both Western and Eastern churches, presbyters are charged with the cure of souls, hence the title Curate for a priest not a rector or vicar.
Insofar as Scripture uses Adam and Christ to represent the ideal of the human race, with the Gospel According to St. John having Pontius Pilate say “Ecce Homo”, there is an inherent compatibility between it and Platonism, and some ideas of Plato were good ideas worth using, and it would change Christianity into something else if we tried to remove anything that had an analogue in Platonic thought; the early Church Fathers were right to use Plato, however, in the case of the Oriental Orthodox vs. the Chalcedonians, the concept of physis was taken too far, and the Oriental Orthodox were falsely accused of the Monophysitism of Eutyches, who Pope Dioscorus had anathematized as a heretic. The Monophysite sect went on to degenerate into Tritheism, since it was the only way they could preserve the idea of Jesus Christ as God while believing that in His nature humanity and divinity were comingled and had been changed into a hybrid in the Incarnation.
Thus we return to the importance of “Fully God, and fully man, without change, confusion, separation or division” the formula that all three Christological groups agree on - the Chalcedonians, who have been accused of Nestorianism by Oriental Orthodox (primarily Ethiopian monks) who believe that saying our Lord abides in two natures is the same as saying He has two hypostases or exists in two persons, and the Miaphysite Oriental Oriental Orthodox who have been accused of Monophysitism by Chalcedonians who fail to recognize that the hymn which the Chalcedonians use to define Christological orthodoxy in a fifth century sense, Only Begotten Son, was composed by St. Severus of Antioch and is the Introit to all Syriac Orthodox liturgies and is used by all Oriental Orthodox churches, and who have also been accused of Monothelitism by Chalcedonians unaware of the fact that the Oriental Orthodox recognized this as a heresy, and it was Monothelitism that is believed to have been the belief harbored by Maronites that lead to their Schism) and the Assyrians of the Church of the East (who have been accused of Nestorianism by both Chalcedonians and Miaphysites who attempt to impose Platonic categories on a Syriac Aramaic word, and based on their history, and also on the unfortunate fact that they continue to venerate Nestorius, a legacy of the period when a Nestorian uncanonically seized control of the Catholicosate of the East, some time before Mar Babai became Catholicos and implemented the current Syro-Chalcedonian Christology).
*Of course later in his pretentious memoirs, the
Bazaar of Heraclides, Nestorius attempted to throw some gasoline onto the fire of schism between the Oriental Orthodox and the Eastern Orthodox, between Alexandria and Rome, for it was Pope St. Cyril of Alexandria and Archbishop St. Celestine of Rome that had anathematized him, and Chalcedon was a clash between Pope Dioscorus of Alexandria and Pontifex Maximus Leo I of Rome (remember, Roman bishops were not yet called Pope; that began in the 6th century, and I am pedantic enough to insist on referring to them using only their styles, but Leo I did adopt the title “Pontifex Maximus” meaning “chief bridge builder” which had previously been used in the now suppressed Roman state religion and which was once held by Gaius Julius Caesar, whose conquests inadvertently facilitated the spread of the Roman Empire (and who interestingly might have been saved by our Lord according to the Pacific belief in the Harrowing of Hell, along with anyone else who died before the Crucifixion, when God remade man in His image on the Sixth day).