Some scholars assert that the Fifth Ecumenical Council in 553 AD at Constantinople resolved Chalcedon's ambiguities regarding the issue of Christ's nature by approving St. Cyril's idea of Miaphysitism, meaning one nature.
There, the Council made a declaration of approval of St. Cyril's phrase "one nature". However, it also anathematized a certain understanding of this phrase, which may depend on how to interpret the Greek meaning of the text, which unfortunately I was unable to find.The Council declared:
IF anyone uses the expression "of two natures," confessing that a union was made of the Godhead and of the humanity, or the expression "the one nature of God the Word incarnate," and shall not so understand those expressions as the holy Fathers have taught, to wit: that of the divine and human nature there was made an hypostatic union, whereof is one Christ; (then he is anathematized).
but [if] from these expressions shall try to introduce one (?) nature or substance of the Godhead and manhood of Christ; let him be anathema.
For in teaching that the only-begotten Word was united hypostatically we do not mean to say that there was made a mutual confusion of natures, but rather each remaining what it was, we understand that the Word was united to the flesh. Wherefore there is one Christ, both God and man, consubstantial with the Father as touching his Godhead, and consubstantial with us as touching his manhood.
First, does the underlined word "one" in Greek say mono or mia? I vagelu remember reading that it said "mono". I have heard that mono is just the male form for the word "one". And indeed the English and Russian translations of this passage do not make any special note that this refers to a homogenous "mono" one.On the other hand,
I know that a major distinction is made between "Monophysites" and "Miaphysites", with the former meaning only a homogenous nature in Christ and the second referring to Christ having a whole, heterogenous, complex nature of His humanity and Divinity.
Another claim, if I understand Fr. Romanides correctly,
seems to be that the ban on the idea of "one nature or substance" is really just talking about a ban on the idea that Christ has just one essence:
In the light of all this and all which was said at Chalcedon, [the earlier, Fourth Council's] anathema pronounced in the definition on those who say 'two natures before the union and one after the union' was intended for anyone with Eutyches who denied that Christ is consubstantial with us. There is no doubt that the definition should have contained the phrase or ousia as one finds after the phrase one physis in the eighth and ninth anathemas of the Fifth Ecumenical Council. This would have avoided much misunderstanding. It perhaps was not done at the Fourth because possibly Cyril's One Nature of God the Logos was taken as equivalent to One Ousia and the word Incarnate as equivalent to a second ousia or physis.
[W]hen in two natures is accepted as the original reading of the Chalcedonian definition (although from two natures is what the rnanuscripts contain), it should be taken as an anti-Eutychianist statement meaning in two ousiajs, since this is what had been denied. Thus the Fifth Ecumenical Council rejects as heretical from two natures only when its proponents mean to teach one ousia in Christ.
Personally, I am skeptical that the phrase "one nature or substance" means "one nature, ie. substance" or "substance, or 'nature' in the sense of substance". For me, nature and substance are definitely different.
Also for me, the phrase "one nature or substance" seems to mean a ban on "either
one nature or substance." But the Council did not say "either", so maybe I am wrong.