St. Cyril expressed a deep Christological intuition within the framework of a still imperfect terminology. In particular, the term "upostasis" was perceived in both Antioch and Alexandria as a synonym for nature (jusis), despite the very accurate use of the word by the great Cappadocians, who used this term in relation to the mystery of the Holy Trinity.

In order to affirm the unity of being, to show that both before and after the Incarnation, the Word is the only Finisher of our salvation, St. Cyril speaks of the one hypostasis or "the one incarnate nature of God the Word" (mia jusis tou Qeou logou sesarkwmenh). The vague term proswpon, used in Antioch to denote the unity of the two natures in Christ, seemed to St. Cyril to be completely unacceptable. Usually he opposed it with the idea of a "hypostatic unity", but because of the synonymous use of the terms upostasis and jusis, adopted by both sides at that time, he was also forced to speak of a "single nature".

For several reasons, the terminology of St. Cyril could not become the basis for the creation of a generally acceptable Christology. First, it did not assign a necessary role to human nature in the matter of salvation. Beginning with Origen, there was a tendency toward "anthropological minimalism," which the Church was forced to restrain, and to which the Monophysites apparently fell a victim. St. Cyril did not realize how much many of his dubious origins were confused by the formula mia jusis tou Qeou logou sesarkwmenh. This formula was borrowed by St. Cyril from a text that he believed to belong to St. Athanasius, but in reality to Apollinarius. The forgery, in which the unsuspecting bishop of Alexandria became an unwitting accomplice, was revealed by Byzantine authors of the sixth century. They, however, did not succeed in preventing the Monophysites from using the ill-fated formula as an anti-Chalcedonian shock weapon. For the Antiochian theologians, who remained faithful to the first anti-Apollinarian polemicists in this, in particular St. Gregory of Nazianzus, formulas such as "the one incarnate nature of God the Word" could not express with sufficient clarity the fullness of the real humanity of Christ. The biblical concept of "flesh" ("the Word was made flesh," John 1:14) in the Greek translation loses its meaning of animate nature and is used mainly as a synonym for the body in its opposition to the soul. The Christology of Apollinaris thus assumed that Christ did not possess a full human nature, but only a human body, and that the place of the rational and immaterial principle in His human nature was taken by the Word, and it was this rational principle that was regarded by the Greeks as the hgemonikon, the governing principle in the human individual. Devoid of human mind (nous), humanity in Christ could not be true human nature, but only an inanimate body in which the Divine Mind dwells.

Although St. Cyril inadvertently used the terminology of Apollinaris in his polemics against the Nestorians, he himself decisively rejected the teaching of Apollinarius. Expressions such as hgemonikon and mia upostasis; seemed to him necessary first of all in order to indicate the identity of the subject between the pre-eternally existing Divine Word and the Incarnate Word. Undoubtedly, St. Cyril fully recognized the fullness of humanity in Christ, the unity of the "two natures." In his first Epistle to Suxens, Cyril lashes out at those who would ascribe to him "the views of Apollinarius" and continues:

If we imagine how the Incarnation took place, we will see that the two natures were united inseparably, inseparably and immutably. At the same time, the flesh remains flesh: it is not the divine nature, although it is the flesh of God, in the same way the Word is God, and not flesh, although in the order of oikonomia He made the flesh His own.

After using such sublime Chalcedonian language, St. Cyril nevertheless returns to his favorite expressions: "We say that there is one Son, as the Fathers said, one incarnate nature of God the Word." It is clear that jusis and upostasis, according to St. Cyril, do not mean "essence." In this case, the essence of the God-Man would be a new essence obtained as a result of the confusion of the Godhead and humanity, the complementary realities from which the hypostasis of the incarnate Word would be composed. This idea of a complex essence, as Diodorus of Tarsus remarks, was the fundamental error of Apollinarius, but we find nothing of the kind in Cyril. Unlike the later Monophysites, however, the great bishop of Alexandria did not require the Antiochians to adopt his own terminology: the phrase "one nature" is not found either in the Anathemas against Nestorius, or in the text of the agreement of 433, which restored unity between the Churches of Antioch and Alexandria.

The victory of St. Cyril in Ephesus (431) resulted not only in the cutting off of the "Nestorian temptation" from Orthodox theology. Its significance lay not only in the affirmation of the truth of the unity of the historical Christ, but also in the proclamation of a positive creative theological conception of the fullness of humanity in Christ, fully assimilated by the Word, a conception affirming the idea of deification, to which all who are "in Christ" are destined. This is what the Council of Ephesus defended when it dogmatized the term "Theotokos" as the name of the Mother of God, because Mary could not be the mother of Christ's "flesh" alone, because this flesh did not have an independent existence, but was truly "the flesh of God." It is in this flesh (kata sarka) that the Son of God suffered, died, and rose again, and through this flesh all redeemed humanity is called to partake of the Holy Spirit. The doctrine of "communion" with the Divinity and the deification of man could not have been created in the context of strictly Antiochian theology.

However, St. Cyril lacked terms that could completely dismiss suspicions of Apollinarianism and make his teaching acceptable to the whole Church. His impetuous mind did not possess the calm clarity with which, fifty years earlier, the great Cappadocians had freed the anti-Arian polemics from verbal confusion and formulated the Nicene faith in terms acceptable to the greater part of the Eastern episcopate. Faithful to the theological system of St. Athanasius (the Apollinarian formulas used by St. Cyril were attributed to St. Athanasius), the bishop of Alexandria was unable or unwilling to apply the Cappadocian definitions of jusis, ousia and upostasis to Christology. This step was to be taken by the Council of Chalcedon, the essential merit of which is the exposition of the teaching of St. Cyril in a theologically accurate and understandable language for Antiochian theologians.

Such certainty was necessary because at the Council of Ephesus the disputes were conducted under exceptional conditions, and the Council's decisions, which were of a polemical nature, only affirmed the Orthodoxy of St. Cyril, while leaving unanswered the real questions posed by Antiochian Christology. In his correspondence with St. Cyril, Nestorius refused to admit that the Word, being God, could "be born" of a Virgin and die on the cross. Nevertheless, he agreed that Christ God, who was made man, was born and suffered in the flesh. Nestorius' teaching was initially ambiguous because it unconditionally assumed the existence of two subjects, Christ and the Word. Obviously, Nestorius' terminology implied that everything that can be said about the Word, regardless of His Incarnation, necessarily refers to the Divine nature itself. In Nestorius' view, expressions such as "the Word who died on the cross" or "born of Mary" detracted from the divine nature, which remained unchanged after the Incarnation. Through interaction with the "temple in which she dwelt" (th tou naou sunajeia), the divine nature of the Word was itself destroyed.

Obviously, only a clear distinction between the Word as a person or hypostasis and the Godhead as a nature, impassible, unchanging, common to all the Persons of the Holy Trinity, could help, if not explain, then at least express the mystery of the Redemption. But neither St. Cyril nor Nestorius had such a distinction. The Church accepted it at the Council of Chalcedon, and this raised a number of new problems that Byzantine theology later tried to solve, but in the middle of the fifth century it seemed that in the Chalcedonian formula the Church had found the only acceptable Orthodox way of expressing the essential features of the mystery of the Incarnation.

The heresy of Eutyches drew the attention of the Eastern Church, still under the impression of the victory of St. Cyril over Nestorius, to the dangers to which triumphant Alexandrism could lead. It was obvious that in the theological dictionary of St. Cyril there were no terms that would allow us to cope with them.

The acts of the Synod of Constantinople, which examined the case of Eutyches under the presidency of Bishop Flavian in the year 448, show that St. Cyril remained the only criterion of Orthodoxy for both the judges and the condemned. Eutyches, who at first did not want to recognize the formula of agreement that St. Cyril had adopted in 433 during his reconciliation with the Easterners (Christ the true God and the true Man and the "unity of the two natures"), finally agreed with it. However, he refused to confess the existence in Christ of two natures "after union," that is, the preservation of the integrity of both natures in Christ. Many bishops who took part in the work of the Synod rightly considered the doctrine of the two natures after union to be Cyrillic in content, although St. Cyril himself never formally affirmed such a thing.

The condemnation of Eutyches seemed to many to be a departure from the teaching of St. Cyril. Dioscorus of Alexandria, with the support of the emperor, easily achieved the deposition of St. Flavian, the rehabilitation of Eutyches, and the condemnation of the consistent defenders of the doctrine of the "two natures after union," namely Bl. Theodoret of Cyrus and Ives of Edessa, who, given their previous friendship with Nestorius, were initially compromised. Such were the deeds of the "robber" Council of Ephesus, which, under the militant leadership of the nephew and successor of St. Cyril on the Alexandrian cathedra, accomplished all this in the name of the theology of St. Cyril. Only the death of Emperor Theodosius II made it possible to change the situation.

The Council of Chalcedon, in which almost all the Eastern bishops participated, numbering 500 (the most representative ecclesiastical forum that the Church had known up to that time), showed that Monophysitism was in principle unacceptable to the Eastern Church, with the exception of Egypt. As soon as the psychological pressure from Theodosius II and Dioscorus disappeared, the episcopate recognized St. Flavian as the spokesman for the catholic faith, and not Dioscorus, and especially not Eutyches. However, no one disputed the absolute authority of St. Cyril, and there was no question of reverting to the Antiochian terminology suspected of Nestorianism in order to condemn heresy. The solution to the problem was found in the arsenal of Western Christology and meant a terminological innovation, a distinction between nature and hypostasis. This distinction, which at that time was not accepted in the East, in Antioch and Alexandria, was a significant creative contribution of Chalcedon to the development of Christological thought. The Council was the most perfect example of "conciliarity" in the history of the Church, which makes it possible to see and formulate in a truly "catholic" language, understandable to all, a truth that has always existed, but which no single local tradition can fully express.