The Ethiopian Orthodox Tewahedo Church was administratively part of the Coptic Orthodox Church of Alexandria from the first half of the 4th century until 1959, when it was granted its own patriarch by Cyril VI, Pope of the Coptic Orthodox Church of Alexandria. It is one of the oldest Christian churches and, as a non-Chalcedonian church, it is not in full communion with the Ethiopian Catholic Church. Ethiopia is the second country historically, following only Armenia, to have officially proclaimed Christianity as state religion (in AD 333).
Tewahedo (Ge'ez ተዋሕዶ) is a Ge'ez word meaning "being made one". This word refers to the Oriental Orthodox belief in the one perfectly unified nature of Christ; i.e., a complete union of the divine and human natures into one nature is self-evident in order to accomplish the divine salvation of humankind, as opposed to the "two natures of Christ" belief commonly held by the Roman Catholic and Eastern Orthodox, Anglican, Lutheran and most Protestant churches. The Oriental Orthodox churches adhere to a Miaphysitic Christological view followed by Cyril of Alexandria, the leading protagonist in the Christological debates of the 4th and 5th centuries, who advocated "mia physis tou theou logou sesarkōmenē", or "one (mia) nature of the Word of God incarnate" (μία φύσις τοῦ θεοῦ λόγου σεσαρκωμένη) and a "union according to hypostasis" (ἕνωσις καθ' ὑπόστασιν henōsis kath' hypostasin), or hypostatic union. The distinction of this stance was that the incarnate Christ has one nature, but that one nature is of the two natures, divine and human, and retains all the characteristics of both after the union.
Miaphysitism holds that in the one person of Jesus Christ, divinity and humanity are united in one (μία, mia - "united") nature (φύσις - "physis") without separation, without confusion, without alteration and without mixing where Christ is consubstantial with God the Father. Around 500 bishops within the Patriarchates of Alexandria, Antioch and Jerusalem refused to accept the dyophysitism (two natures) doctrine decreed by the Council of Chalcedon in 451, an incident that resulted in the first major split in the main body of the Christian Church.
Tewahedo (Ge'ez: ተዋሕዶtäwaḥədo) is a Ge'ez word meaning "being made one" or "unified". This word refers to the Oriental Orthodox belief in the one single unified nature of Christ; i.e., a belief that a complete, natural union of the Divine and Human Natures into One is self-evident in order to accomplish the divine salvation of humankind. This is in contrast to the "two Natures of Christ" belief (unmixed, but unseparated Divine and Human Natures, called the hypostatic union) which is held by the Catholic Church and the Eastern Orthodox Church.
Oriental Orthodoxy is known as "non-Chalcedonian", and, sometimes by outsiders as "monophysite" (meaning "One Single Nature", in allusion to Jesus Christ). However, these Churches themselves describe their Christology as miaphysite (meaning "one united nature" in reference to Jesus, the Greek equivalent of "Tewahedo").