Oriental Orthodoxy is the faith of those Christian churches which as a rule recognize only the first three ecumenical councils – the First Council of Nicaea, the First Council of Constantinople, and the First Council of Ephesus. The Assyrian church (The Church of the East) recognizes the First Council of Nicaea and the First Council of Constantinople as legitimate ecumenical councils, other councils being the respected councils of its sister Latin church and Eastern Orthodox churches in the West. There are approximately 84 million adherents worldwide.

They rejected the definitions of the Council of Chalcedon held in AD 451 in Chalcedon. Hence, these Oriental Orthodox churches are also called Old Oriental churches, and, with the exception of the Assyrian church (which is Nestorian), Miaphysite churches, or the Non-Chalcedonian churches, known to Western Christianity and much of Eastern Orthodoxy as Monophysite churches (although the Oriental Orthodox themselves reject this description as inaccurate, having rejected the teachings of both Nestorius and Eutyches). These churches are in full communion with each other but not with the Eastern Orthodox churches. Slow dialogue towards restoring communion began in the mid-20th century.