The Chalcedonian Definition (also Confession or Creed of Chalcedon) was adopted in 451 at the Council of Chalcedon in Asia Minor. That council was the fourth of the first seven Ecumenical Councils, which are accepted by Chalcedonian churches (Eastern Orthodox, Catholic, and many Protestant churches). It is the first Council not recognized by any of the Oriental Orthodox churches, which may be classified as non-Chalcedonian.
The Definition defines that Christ is ‘acknowledged in two natures’, which ‘come together into one person and one hypostasis‘. The formal definition of ‘two natures’ in Christ was understood by the critics of the council at the time, and is understood by many historians and theologians today, to side with western and Antiochene Christology and to diverge from the teaching of Cyril of Alexandria, who always stressed that Christ is ‘one’. However, a modern analysis of the sources of the creed (by A. de Halleux, in Revue Theologique de Louvain 7, 1976) and a reading of the acts, or proceedings, of the council (recently translated into English) show that the bishops considered Cyril the great authority and that even the language of ‘two natures’ derives from him.