The Chalcedonian Definition is a declaration of Christological orthodoxy that emerged from the Council of Chalcedon, convened in 451 AD. This council was one of the four early Ecumenical Councils recognized by the Eastern Orthodox Church, the Catholic Church, and most Protestant denominations as having authority in doctrinal matters.
Following the First Council of Ephesus in 431, where Nestorianism was condemned, a new controversy arose over the nature of Christ, leading to the formation of the Eutychianism or Monophysitism, which suggested that Christ had only one nature, a divine one, that absorbed or was transformed by His humanity.
The Council of Chalcedon sought to address this issue by affirming the orthodox understanding of the Incarnation. The Definition, also known as the Definition of Faith, states:
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.
The Chalcedonian Definition was accepted by the majority of the Church, but it led to significant schisms:
Today, the Chalcedonian Definition remains a foundational document for understanding the doctrine of the Incarnation within many Christian traditions. It continues to be a point of reference in theological discussions about Christ's nature.