The early Christians labored to defend the biblical faith against heresies, none more destructive that the Nestorian, Arian and Eutychian assaults on the doctrine of the Incarnation of Christ, God made flesh. At the Council of Nicea, and later at the Council of Chalcedon, the orthodox, biblical faith was stated clearly and unequivocally. The Definition of the Council of Chalcedon was adopted in 451 AD, proclaiming the true humanity and true divinity of our Lord and Savior, Jesus Christ.
Therefore, following the holy fathers, we all with one accord teach men to acknowledge one and the same Son, our Lord Jesus Christ, at once complete in Godhead and complete in manhood, truly God and truly man, consisting also of a reasonable soul and body; of one substance with the Father as regards his Godhead, and at the same time of one substance with us as regards his manhood; like us in all respects, apart from sin; as regards his Godhead, begotten of the Father before the ages, but yet as regards his manhood begotten, for us men and for our salvation, of Mary the Virgin, the God-bearer; one and the same Christ, Son, Lord, Only-begotten, recognized in two natures, without confusion, without change, without division, without separation; the distinction of natures being in no way annulled by the union, but rather the characteristics of each nature being preserved and coming together to form one person and subsistence, not as parted or separated into two persons, but one and the same Son and Only-begotten God the Word, Lord Jesus Christ; even as the prophets from earliest times spoke of him, and our Lord Jesus Christ himself taught us, and the creed of the fathers has handed down to us.