And so there developed along the Euphrates a fourfold border: political, dogmatic, ecclesiastical and linguistic.
Since the Church of the East was denied access to the West, it consequently oriented itself towards the East. While Bishop David of Basra initiated contact with the Indian Thomas Christians of Kerala around 295/300, Nestorian monastic missionaries advanced into the Arabian Peninsula, as well as towards the peoples of the Central Asian steppes. After the loss of its Arab dioceses to Islam and a first setback in China, the Church made renewed efforts towards the east beginning in the eleventh century, and reached the Mongol peoples and the Middle Kingdom.
At that time, the authority of the patriarch of the Church of the East extended from the Euphrates to the Yellow Sea…Baumer, Church of the East, p. 3