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In classical Christian usage, the term orthodox refers to the set of doctrines which were believed by the early Christians. A series of ecumenical councils were held over a period of several centuries to try to formalize these doctrines. The most significant of these early decisions was that between the Homoousian doctrine of Athanasius and Eustathius (which became Trinitarianism) and the Heteroousian doctrine of Arius and Eusebius (called Arianism). The Homoousian doctrine, which defined Jesus as both God and man with the canons of the 431 Council of Ephesus, won out in the Church and was referred to as orthodoxy in most Christian contexts since this was the viewpoint of previous Christian Church Fathers and was reaffirmed at these councils. (The minority of nontrinitarian Christians object to this terminology).
Following the 1054 Great Schism, both the Western Church and Eastern Church continued to consider themselves uniquely orthodox and Catholic. Augustine wrote in On True Religion: “Religion is to be sought . . . only among those who are called Catholic or Orthodox Christians, that is, guardians of truth and followers of right.” Over time, the Western Church gradually identified with the “Catholic” label, and people of Western Europe gradually associated the “Orthodox” label with the Eastern Church (in some languages the “Catholic” label is not necessarily identified with the Western Church). This was in a note of the fact that both Catholic and Orthodox were in use as ecclesiastical adjectives as early as the 2nd and 4th centuries respectively.
Much earlier, Oriental Orthodoxy had split from Chalcedonian Christianity after the Council of Chalcedon (AD 451), because of several Christological differences. Since then, Oriental Orthodox Churches are maintaining the orthodox designation as a symbol of their theological traditions.