What does the Chalcedonian definition of faith say ?
[Translation of the Chalcedonian creed adopted at Session V, from the Acts of the Council of Chalcedon]
Following the holy Fathers we teach with one voice that the Son [of God] and our Lord Jesus Christ is to be confessed as one and the same [Person], that he is perfect in Godhead and perfect in manhood, very God and very man, of a reasonable soul and [human] body consisting, consubstantial with the Father as touching his Godhead, and consubstantial with us as touching his manhood; made in all things like us, sin only excepted; begotten of his Father before the worlds according to his Godhead; but in these last days for us men and for our salvation born [into the world] of the Virgin Mary, the Mother of God according to his manhood. This one and the same Jesus Christ, the only-begotten Son [of God] must be confessed to be in two natures [duo physis], unconfusedly, immutably, indivisibly, inseparably [united], and that without the distinction of natures being taken away by such union, but rather the peculiar property of each nature being preserved and being united in one Person and subsistence, not separated or divided into two persons, but one and the same Son and only-begotten, God the Word, our Lord Jesus Christ, as the Prophets of old time have spoken concerning him, and as the Lord Jesus Christ has taught us, and as the Creed of the Fathers has delivered to us.
Another translation:
We then following the Holy Fathers all with one accord teach men to confess one and the same Son, our Lord Jesus Christ; perfect in Godhead and perfect in manhood, truly God and at the same time truly man with a reasonable soul and body; consubstantial with the Father according to his Godhead and consubstantial with us according to his manhood, in all things like unto us apart from sin, begotten both before all ages of the Father according to his Godhead and in the last days born of Mary the Virgin, the theotokos for us and our salvation, according to his manhood, one and the same Christ, Son, Lord, only begotten, being made known in two natures without confusion [unconfusedly asugehutos], without conversion [immutably atreptos], without division [indivisibly adiairetos], without separation [inseparably achoristos], the distinction of natures having been in no way abolished through the union, but rather the property of each nature being preserved and meeting in one person and one hypostasis. This Jesus Christ is the only begotten Son, the divine Logos and the Lord. It was as it was told by prophets from the beginning, taught by Jesus Christ himself and proclaimed by the creed of the Holy Fathers.
What all do we find acceptable?
Four terms used in the above definition is against the teachings of some heretics and are completely acceptable to us Oriental Orthodox.
1. unconfusedly [asugehutos] : Unconfusedly means without confusion. This is against the heresy of Eutychianism. Eutyches taught that the human nature of Christ “fused” with the divine nature as drop of vinegar is consumed by the ocean.
2. Immutable [atreptos]: The dictionary meaning of immutable is "unchanging over time". This is used against the heresy of Appolinarianism. Appolinaris taught that Christ had a human body (soma) and human soul (psyche) but not a human rational mind (nous). The Logos Himself is, or takes the place of, the human spirit, thus becoming the rational and spiritual center, the seat of self-consciousness and self-determination.
3,4 indivisibly [adiairetos] and inseparably [achoristos] : These terms are used against the heresy of Nestorianism. Nestorius stressed the distinction between the divinity and the humanity in Christ to such an extent that it appeared that two persons were living in the same body.
What do we find unacceptable ?
The term in two natures creates problems and is unacceptable. It is very close to the heresy of Nestorius. In fact the terms indivisibly [without division] and inseparably [without separation] that are used against Nestorianism becomes meaningless if we confess Christ to be in two natures. Moreover it is incomprehensible that two natures will exist in one person. Nestorianism profess that Christ was human when he slept and He was God when he rebuked the wind and sea (Mark 4:38-39). It is incomprehensible when it is said that when Jesus wept at the tomb of Lazarus he was man and when he raised Lazarus from the dead he was God. This incomprehensibility can be removed if we say 'from two natures' instead of 'in two natures'. Oriental Orthodox Churches teach us that Christ is One Person from two natures.
We hold on to our Mia-Physis (One Nature) Christology of Mor Koorilose (St. Cyril of Alexandria) who said famously: "mía phýsis toû theoû lógou sesarkōménē" translated as "one (mia) nature of the Word of God incarnate" . Our Christology is further clarified by our father among the saints Mor Severious (St.Severus of Antioch) who said "We must confess the difference between the two natures from which the one Christ is, and avoid the cutting into two, and extol one Son and Christ, and one incarnate nature of God the Word."