Chalcedonian Christianity refers to the Christian denominations adhering to the christological definitions and ecclesiological resolutions of the Council of Chalcedon, the Fourth Ecumenical Council held in 451. Chalcedonian Christians follow the Definition of Chalcedon, a religious doctrine concerning the divine and human natures of Jesus Christ. The great majority of Christian communions and confessions in the 21st century are Chalcedonian, but from the 5th to the 8th centuries the ascendancy of Chalcedonian Christology (detailed below) was not always certain.
Doctrinally, Chalcedonianism may be regarded as a subset of Nicene Christianity.
Not shown: non-Nicene, nontrinitarian, and some restorationist denominations. Additionally, the diagram above is simplified without depiction of crypto-Protestants within Catholicism, crypto-Catholics and Protestant Eastern Christianity in Protestantism, or interdenominational movements such as Pietism and the Charismatic Movement.
The dogmatic disputes raised during the Council of Chalcedon led to the Chalcedonian Schism thus to the formation of the Non-Chalcedonian body of churches known as Oriental Orthodoxy. The Chalcedonian churches remained united with the Holy See of Rome, the Ecumenical Patriarchate of Constantinople (or "New Rome") and the Eastern Orthodox patriarchates of the Middle East (namely Alexandria, Antioch and Jerusalem). Together, these five patriarchates were considered the pillars of orthodox catholic Christendom and of the Chalcedonian confession of faith. During the 6th-century reign of Emperor Justinian I, the five patriarchates were recognised as the Pentarchy, the official ecclesiastical authority of the Imperial Christian Church.
Today, the great majority of Christian denominations can be considered descended from the Pentarchy, subscribing to Chalcedonian Christianity, broadly divided into the Roman Catholic Church in the predominantly Latin-speaking West, the Eastern Orthodox Church in the predominantly Greek-speaking East, and the Protestant denominations created in the wake of the Protestant Reformation.
The groups that rejected Chalcedon's Christological definition were the majority of the Armenian, Coptic, and Ethiopian Christians, together with a part of the Indian and Syriac Christians (the latter of which came to be identified as Jacobites). Today, such groups are known collectively as the Non-Chalcedonian, Miaphysite, or Oriental Orthodox churches.
Some Armenian Christians, especially in the region of Cappadocia and Trebizond inside the Byzantine Empire, accepted the decisions of the Council of Chalcedon and engaged in polemics against the Armenian Apostolic Church.
After the conclusion of Byzantine-Sasanian War of 572–591, direct rule of the Byzantine Empire was extended to all western parts of Armenia, and soon after that emperor Maurice (582–602) decided to strengthen his political control over the entire region by supporting the local pro-Chalcedonian faction of the Armenian Church. In 593, a regional council of western Armenian bishops was convened in the city of Theodosiopolis, and proclaimed allegiance to the Chalcedonian Definition.
Those present at the Council of Chalcedon accepted Trinitarianism and the concept of hypostatic union, and rejected Arianism, Modalism, and Ebionism as heresies (which had also been rejected at the First Council of Nicaea in AD 325). Those present at the council also rejected the Christological doctrines of the Nestorians, Eutychians, and monophysites (these doctrines had also been rejected at the First Council of Ephesus in 431).
The Chalcedonian understanding of how the divine and human relate in Jesus Christ is that the humanity and divinity are exemplified as two natures and that the one hypostasis of the Logos perfectly subsists in these two natures. The Non-Chalcedonians hold the position of miaphysitism (sometimes called monophysitism by their opponents). Miaphysitism holds that in the one person of Jesus Christ, divinity and humanity are united in one nature, the two being united without separation, without confusion and without alteration. That led many members of the two churches to condemn each other: the Chalcedonians condemning the Non-Chalcedonians as Eutychian Monophysites, and the Non-Chalcedonians condemning the Chalcedonians as Nestorians.
Later interpreters of the council held that Chalcedonian Christology also rejected monothelitism and monoenergism (rejected at the Third Council of Constantinople in 680). Those who did not accept the Chalcedonian Christology now call themselves non-Chalcedonian. Historically, they called themselves Miaphysites or Cyrillians (after St Cyril of Alexandria, whose writing On the Unity of Christ was adopted by them and taken as their standard) and were called by Orthodox Christians monophysites. Those who held to the non-Chalcedonian Christologies called the doctrine of Chalcedon dyophysitism.