On October 8, 451 the largest of all church councils opened at Chalcedon, near Constantinople (modern Istanbul). There were 500-600 bishops representing numerous conflicting views on the nature and person of Jesus Christ in the church.
On this date, October 22, 451, the Chalcedon Creed was adopted which affirmed the Nicene (325)-Constantinople (381) Creed that stated Jesus was fully God and fully man. At Chalcedon it was also affirmed that the two natures (fully/true God and fully/true man) subsisted in one person, Jesus Christ. Here is the complete statement:
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 [theotokos]; 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 [hypostasis], 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.
This had been preceded by Leo’s, Bishop of Rome (440-461), statement in what is known as The Tome of Leo, June 13, 449:
Thus the properties of each nature and substance were preserved entire, and came together to form one person. Humility was assumed by majesty, weakness by strength, mortality by eternity; and to pay the debt that we had incurred, an inviolable nature was united to a nature that can suffer. And so, to fulfill the conditions of our healing, the man Jesus Christ, one and the same mediator between God and man, was able to die in respect of the one, unable to die in respect of the other. Thus there was born true God in the entire and perfect nature of true man, complete in his own properties, complete in ours.
The key biblical truth affirmed in the Chalcedonian Creed is that Jesus Christ is fully God and fully man, two natures, one person without becoming more or less human or divine. Christ’s work is the work of one person, not of one nature or another. Christological heresies occur by confusing this central and essential truth.