After this split with the Western World and adoption of Nestorianism, The Church of the East expanded rapidly due to missionary works during the Medieval period.During the period between 500–1400 the geographical horizons of the Church of the East extended well beyond its heartland in present-day northern Iraq, north eastern Syria and south eastern Turkey.
For its members as an ethnoreligious group, see Chaldean Catholics.
In 2015, while the patriarchate of the Assyrian Church of the East was vacant following the death of Dinkha IV, the Chaldean Catholic Patriarch Louis Raphaël I Sako proposed a "merger", or reunion, of the Chaldean Catholic Church with the other denominations that trace their origins to the Church of the East: the Assyrian Church of the East and the Ancient Church of the East, in order to recreate one united "Church of the East" with a single patriarch in full communion with the Pope.
The Chaldean Catholic Church traces its beginnings to the Church of the East, which it considers to have been founded between the 1st and 3rd centuries in Asōristān, a province of the Sasanian Empire.
Saint Peter, chief of the apostles, added his blessing to the Church of the East at the time of his visit to the See at Babylon in the earliest days of the church when stating, "The elect church which is in Babylon, salutes you; and Mark, my son." (1 Peter ).
Although considered founded in the 1st century by the adherents of its legacy, the Church first achieved official state recognition from Sasanian Iran in the fourth century with the accession of Yazdegerd I (reigned 399–420) to the throne of the Sasanian Empire.
The Council condemned as heretical the Christology of Nestorius, whose reluctance to accord the Virgin Mary the title Theotokos "God-bearer, Mother of God" was taken as evidence that he believed two separate persons (as opposed to two united natures) to be present within Christ.