Chalcedon
The site of Chalcedon is located on a small peninsula on the north coast of the Sea of Marmara, near the mouth of the Bosphorus. A stream, called the Chalcis or Chalcedon in antiquity and now known as the Kurbağalıdere (Turkish: "stream with frogs"), flows into Fenerbahçe bay. There Greek colonists from Megara in Attica founded the settlement of Chalcedon in 685 BC, some seventeen years before Byzantium.
Christology
The 451 Council of Chalcedon was highly influential and marked a key turning point in the Christological debates that broke apart the church of the Eastern Roman Empire in the fifth century. It is the last council which many Anglicans and most Protestants consider ecumenical. It fully promulgated the Western dyophisite understanding put forth by Pope Leo I of Rome of the hypostatic union, stating the human and divine natures of Christ coexist after the union, yet each is distinct and complete. Most importantly, it unquestionably established the primacy of Rome in the East over those who accepted the Council of Chalcedon. This was reaffirmed in 519 when those Eastern Chalcedonians accepted the Formula of Hormisdas anathematizing all of their own Eastern Chalcedonian hierarchy who died out of communion with Rome from 482-519. Although, the Chalcedonian Creed did not put an end to all Christological debate, it did clarify the terms used and became a point of reference for many future Christologies. Most of the major branches of Western Christianity – Roman Catholicism, Eastern Orthodoxy, Anglicanism, Lutheranism, and Reformed – subscribe to the Chalcedonian Christological formulation, while many branches of Eastern Christianity - Syrian Orthodoxy, Assyrian Church, Coptic Orthodoxy, Ethiopian Orthodoxy, and Armenian Apostolicism – reject it.