The Assyrian Church
The Assyrian Church was one of the earliest to separate itself from communion with the Catholic Church. It traces its origins to the See of Seleucia-Ctesiphon, supposedly founded by Saint Thomas the Apostle as well as Saint Mari and Saint Addai as asserted in the Doctrine of Addai. This church is sometimes referred to as the "Nestorian Church", the "Syrian Church" or the "Persian Church."
It has also been referred to, inaccurately, by a number of other names. These include Assyrian Orthodox Church, which has led some to mistakenly believe that it is a body 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 although the church itself considers the term pejorative. The church declares that no other church has suffered as many martyrdoms as the Assyrian Church of the East.
The Assyrian Church is the original Christian church in what was once Parthia; eastern Iraq and Iran. Geographically it stretched in the medieval period to China and India: inscriptions in Chinese and Syriac on a monument found in Xi'an (Hsi-an), the Tang-period capital of China (originally Chang'an), describe 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. Prior to the Portuguese arrival in India in 1498, it provided "East Syrian" bishops to the Saint Thomas Christians. Patriarch Timothy I (780-820/23) wrote of the large Christian community in Tibet.