The Chalcedonian Creed 451 AD: “The Chalcedonian Definition”
"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."
Notes:
The Council of Chalcedon met in the city of Chalcedon in AD 451. The council’s ruling on the hypostatic union was an important step in further clarifying the nature of Christ and the orthodox doctrine of the Trinity. The council also laid the groundwork for what would become one of the most significant events in ecclesiastical history—the Great Schism (really one more data point leading to the schism).
This council falls in line with previous councils in Nicea (325 AD) and Constantinople (381 AD), which combated heretical views concerning the nature of Jesus. Arius argued for the false doctrine that the Son of God was a created being and that He was of a different substance (heteroousios) than the Father. Apollinaris, argued for the false doctrine that said Jesus’ divine nature had displaced His human mind and will. According to Apollinaris, Jesus was not fully human. Later, Nestorius said Jesus had two separate natures and two wills, essentially making Him two persons sharing one body.
The Council produced the “Chalcedonian Definition,” which affirms that Christ “is at once complete in Godhead and complete in manhood, truly God and truly man.” He is consubstantial [homoousios] with the Father according to the Godhead, and consubstantial with us according to the Manhood. Jesus Christ is to be acknowledged in two natures, inconfusedly, unchangeably, indivisibly, and inseparably. The divine and human natures of Christ are distinct yet united in one Person. This co-existence of Christ’s two natures is called the hypostatic union.
By affirming that Jesus Christ is one Person who is both divine and human, the Council of Chalcedon made it easier to identify error and heresy. The Chalcedonian Definition affirms the truth that Jesus Christ is fully divine and, at the same time, fully human.