The Chalcedonian Definition addresses whether Jesus has one or two natures. It was written and adopted at the Council of Chalcedon in AD 451. This was the first church council that was not recognized by the Oriental Orthodox church.
Eutyches, who was archimandrite at Constantinople, believed that Jesus possesses only one nature. He was able to successfully argue for his position at the Second Council of Ephesus in 449, which is sometimes called the “Robber Synod.” This led to a response from the Council of Chalcedon, which produced the Chalcedonian Definition, or Chalcedonian Creed.
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.