"The Word became flesh and made His dwelling among us. We have seen his glory, the glory of the One and Only who came from the Father, full of grace and truth" (John 1:14).
The Council of Nicaea had affirmed that the Son of God is fully God and not a creature. The church still had to work out what it meant for the incarnate Son of God to be fully man in the God-man Jesus Christ. A variety of inadequate and heretical options were offered in the early centuries.
Apollinarius advocated the heresy known as Apollinarianism. He said that the Logos of the Son took the place of the human soul in the man Jesus Christ. Christ’s physical body, he said, was a true human body, but His deepest soul was divine and He did not have a human soul. The church answered that this denied the total and true humanity of Jesus Christ.
Nestorius championed the heresy known as Nestorianism. He denied that there was a real unity of two natures in Jesus Christ, and said rather that there was simply a juxtaposition. There was a close relationship between God and man in Christ, but not a union. There were two persons in Jesus existing in a unique moral fellowship. The church answered that this denied the true unity of God and man in the incarnation. The church stated that the two natures of Christ exist in unconfused union.
Eutyches promoted the heresy known as Eutychianism or Monophysitism. His idea was similar to Apollinarius’ but the opposite of Nestorius’. He said that Christ had one nature, which merged divine and human nature. The church replied that Christ had to be fully divine, not partially so, in order to save us, and that He had to be fully human, not partly human, in order to represent us.
Each of these heresies really attacked the uniqueness of Jesus. At the Council of Chalcedon, the church stated that Christ is true God and true man, and that these two natures exist in one Person without confusion, without conversion, without division, and without separation.
The Confession of Chalcedon provides a clear and important statement on the human and divine nature of Christ:
We, then, following the holy Fathers, all with one consent, teach people to confess one and the same Son, our Lord Jesus Christ, the same perfect in Godhead and also perfect in manhood; truly God and truly man, of a reasonable [rational] soul and body; consubstantial [co-essential] with the Father according to the Godhead, and consubstantial with us according to the Manhood; in all things like unto us, without sin; begotten before all ages of the Father according to the Godhead, and in these latter days, for us and for our salvation, born of the Virgin Mary, the Mother of God, according to the Manhood; one and the same Christ, Son, Lord, only begotten, to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, inseparably; (ἐν δύο φύσεσιν ἀσυγχύτως, ἀτρέπτως, ἀδιαιρέτως, ἀχωρίστως – in duabus naturis inconfuse, immutabiliter, indivise, inseparabiliter) the distinction of natures being by no means taken away by the union, but rather the property of each nature being preserved, and concurring in one Person (prosopon) and one Subsistence (hypostasis), not parted or divided into two persons, but one and the same Son, and only begotten God (μονογενῆ Θεόν), the Word, the Lord Jesus Christ; as the prophets from the beginning [have declared] concerning Him, and the Lord Jesus Christ Himself has taught us, and the Creed of the holy Fathers has handed down to us.