Assyrian church in northern Iraq
The Assyrian Church of the East also called Holy Apostolic Catholic Assyrian Church of the East is a Christian church. It is one of the earliest churches to separate from the larger Church. It traces its origins to the See of Seleucia-Ctesiphon, started by Saint Thomas the Apostle as well as Saint Mari and Addai as shown in the Doctrine of Addai. This church is sometimes known as the "Nestorian Church", the "Syrian Church" or the "Persian Church." The church is currently headed by Mar Dinkha IV.
Other names have been given as well, but they are not accurate. One of these names is Assyrian Orthodox Church, which has led some people to believe that it is part of the Oriental Orthodox community. The church itself does not use the word "Orthodox" in any of its service books or in any of its official correspondence, nor does it use any word which can be translated as "correct faith" or "correct doctrine", the rough translation of the word Orthodox. In India, it is known as the Chaldean Syrian Church. In the West it is often known as the Nestorian Church. The Church itself believes that the term 'Nestorian Church' is chosen badly.
The Assyrian Church is the original Christian church in what was once Parthia; eastern Iraq and Iran. Geographically its influence stretched to China and India in the Middle Ages: a monument found in Xi'an (Hsi-an), the Tang-period capital of China (originally Chang'an), in Chinese and Syriac described the activities of the church in the 7th and 8th century, while half a millennium later a Chinese monk went from Beijing to Paris and Rome to call for an alliance with the Mongols against the Mamelukes. Before the Portuguese arrival in India in 1498, it provided "East Syrian" bishops to the Saint Thomas Christians. Patriarch Timothy I (727–823) wrote of the large Christian community in Tibet.
The creators of Assyrian theology are Diodorus of Tarsus and Theodore of Mopsuestia. Both taught at Antioch. The normative Christology of the Assyrian church was written by Babai the Great (551–628) and is clearly different from the accusations of dualism directed toward Nestorius: his main christological work is called the 'Book of the Union', and in it Babai teaches that the two qnome (essences) are unmingled but everlastingly united in the one parsopa (personality) of Christ.