By 434, the controversy with Nestorianism had been settled with a definition of union that clarified the orthodox doctrine. This definition teaches two natures in Christ. Nevertheless, because Cyril preached "one incarnate nature", the Alexandrians, led by their bishop Dioscorus (he succeeded Cyril in 444), affirmed that the two natures of Christ became one nature after the Incarnation. This was called Eutychianism, after the monk Eutyches of Constantinople, who in 448 complained to Pope St. Leo that Nestorianism had been revived, because he mistook the assertion of two natures for the Nestorian heresy. The followers of Eutyches later became known as Monophysites, since they asserted that Christ had one nature.
St. Flavian, the Bishop of Constantinople,
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Also, the Council affirms that consubstantial with us in our humanity does not exclude the fact that Christ was unlike us in regards to sin. St. Leo points out that, Christ 's sinlessness does not separate from his humanity, however, it perfects it.
The doctrine of the Incarnation expressed in the Nicene Creed is affirmed and explained, as the Son is eternally begotten from the Father with regards to his divinity, but also born of the Virgin Mary with regards his humanity, because he is one and the same Christ, the Virgin deserves to be called "God-bearer" (Theotokos).
Therefore, because Christ is the only-begotten Son of God, there is only one Son in both the human and divine natures. The two natures (being the essences of what it is to be human or divine), do not change because One person exists in both of them. The human nature and the divine nature are distinct and unique. However, a single Person (prosopon) and subsistent being (hypostasis) was able to adopt the properties of both natures as His own.
...he is not parted or divided into two persons, but is one and the same only-begotten Son, God, Word, Lord Jesus Christ, just as the prophets taught from the beginning about him, and as the Lord Jesus Christ himself instructed us, and as the creed of the fathers handed