The history of the Nestorian Christian sect goes back to the period of Sassanid rule in Iraq, with fewer participants than adherents of the Jacobite sect, given that Al-Mada'in, the capital of the Sassanian state, was a center for the Nestorians, which was later known as the position of the Catholicos, even after the control of the Abbasid state, because Baghdad was its capital.The Catholicos was responsible for appointing and transferring bishops and metropolitans from all the countries under his jurisdiction, which in the late 5th century AH included Syria, Palestine, Egypt, Iran, and China. In the late Abbasid period, the Catholicos was given a charter from the Abbasid caliph that made his authority official over his flock, and required them to obey him and judge them in accordance with their own laws.The conditions of the Christians of Iraq under the Mongol Ilkhans were not uniform, but changed with the change of rulers. Their kings, who were Christians themselves, treated their leaders well and exempted their subjects from the jizya (poll tax) and granted them charters confirming this. Hulegu Khan supported the Christians because his wife was a Christian. The Mongols' care for the Christians went so far that some of the Ilkhans or their representatives participated in the installation of their supreme religious leader, the Catholicos, in official ceremonies held under their patronage. We find that the election of the Catholicos in the Ilkhanid period was first carried out by the nomination of Christian leaders. The Catholicos played a major role in the Ilkhanid Mongol state by mitigating some of the punishments imposed on Christian subjects by Mongol rulers.