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.

Western theology, as can be seen from Pope Leo's famous Epistle to Flavian, had the obvious advantage over the Alexandrian and Antiochian systems in that it affirmed the perfect reality of the two natures (substances) in Christ, while having nothing in common with Nestorianism. The tendency of Latin thought, already manifested in Tertullian, that Christ was regarded primarily as the Mediator between God and humanity, was rather juridical, based more on the idea of sacrifice and reconciliation than on the Eastern idea of deification. However, this concept would not be possible without the recognition that Christ is both God and Man, but at the same time one Person.

The result of Pope Leo's first intervention in Eastern affairs gave the Antiochian theologians a chance of survival at a time when they were in danger of being swept away by the Monophysite wave. In spite of this, the basic essence of St. Cyril's teaching on the unity of Christ was not abandoned, nor was the central idea of Eastern soteriology the idea of communion or communion of mankind with the Divinity, which was central. The goal of Pope Leo was not abstract reasoning about how to properly understand the unity of the two natures in Christ, but the restoration of common sense in the interpretation of Scripture, where Jesus clearly appears both as God and as Man. Eutyches, who, according to Leo, was multum inprudens et nimis imperitus ("very reckless and excessively inexperienced") clearly departed from this common sense. The Gospel text clearly shows us that each of the natures of the incarnate Word retains its own action (agit utraque forma quod proprium est), but each acts only in interaction with the other (alterius communione). This interaction is not a simple juxtaposition, but a union based on the unity of the subject of divine and human action in Christ: One and the same is both God and Man (qui enim verus est Deus, idem verus est homo). Although St. Leo does not speak directly about deification, the essential theological prerequisites of this teaching are implied in him. In the final analysis, Leo's text contains a clear conception of communicatio idiomatum, that is, the very thing that was a stumbling block for the "Nestorian" Antiochian theologians: the unity of the person allows us to say that "the Son of God died" (unitatem personae in utraque natura intelligendam Filius Dei crucifixus dicitur et sepultus), without the divine nature losing its inherent impassibility. This was the very moment because of which St. Cyril was forced to fight with Nestorius: God, without ceasing to be God, makes human nature His own, even to its mortality.

Тем не менее, латинская терминология Льва не могла удовлетворить Восток. Тринитарные споры IV века уже показали, к каким недоразумениям может привести параллельное употребление терминов persona (proswpon) и substantia — natura (ousia — jusis). Поэтому на Халкидонском Соборе термин Льва persona был переведен как upostasis, и таким образом был положен конец двусмысленному употреблению термина Св. Кирилла mia jusis(«единая природа»). Вот знаменитый текст халкидонского вероопределения:

Одного и Того же Христа, Сына Господа, единородного; в двух естествах неслитно, неизменно, нераздельно, неразлучно познаваемого, — так что соединением нисколько не нарушается различие двух естеств, но тем более сохраняется свойство каждого естества и соединяется в одно лицо proswpon и одну ипостась, — не на два лица рассекаемого или разделяемого, но одного и Того же Сына и единородного Бога–Слова, Господа Иисуса Христа, как издревле пророки и как Сам Господь Иисус Христос научил нас и как передал нам символ Отцов наших.

Составленное на основании формулы согласия (433 г.) и исповедания Флавиана (448 г.), халкидонское вероопределение явилось результатом ряда труднейших компромиссов между противоборствующими сторонами. Тем более поразительно в нем положительное разрешение христологической проблемы.

Подавляющее большинство отцов на Соборе определенно были сторонниками св. Кирилла, и в первом варианте представленного Собору текста просто повторялось определение 433 г.: «из двух природ» (ek duw jusewn), — кириллова формула, которую монофизиты готовы были принять, так как она позволяла им говорить о «единой природе после соединения». Термин «природа» сохранял, таким образом, свое старое значение конкретного существа, являясь синонимом термина «ипостась», при этом двусмысленность александрийской терминологии оставалась прежней. Это явилось причиной ультиматума папских легатов и вызвало давление со стороны представителей императора, принуждавших отцов Собора отправить текст на доработку в комиссию. Принципиально антинесторианский, Собор не представлял, каким образом сформулировать свою антимонофизитскую позицию, остававшуюся лишь неясным ощущением.

Тем не менее, окончательное и единодушное принятие последнего варианта нельзя объяснить простым внешним давлением. Это не было капитуляцией перед Римом, как пытались представить дело монофизиты, не было это и отказом от богословия св. Кирилла.