Which council in 451 defined the Chalcedonian doctrine followed by Byzantium?

The council that defined the Chalcedonian doctrine followed by Byzantium was the Council of Chalcedon, held in 451. It met at Chalcedon, a city across the Bosporus from Constantinople, during the reign of Emperor Marcian.

The council addressed fierce disputes about the nature of Christ. Its Definition of Faith taught that Christ is fully divine and fully human, united in one person without confusion, change, division, or separation. This formulation became known as Chalcedonian Christianity and was accepted by the Byzantine imperial church, as well as by the Catholic and Eastern Orthodox traditions.

The council followed the controversy surrounding the Second Council of Ephesus in 449, which many opponents later rejected as illegitimate. Chalcedon also reaffirmed earlier creeds and accepted the Tome of Pope Leo I as an important statement of orthodox Christology.

The main historical complication is that not all Christian communities accepted Chalcedon. The Oriental Orthodox churches rejected its definition and developed distinct theological traditions. That disagreement became one of the enduring religious divisions of the late Roman and Byzantine worlds.