[12] That turbulent period was marked by several consequent splits and mergers, resulting in the creation of separate branches and rival patriarchal lines.

By the end of 1552, a pro-Catholic party had been organized in Mosul under the leadership of the priest Yohannan Sulaqa,[15] who decided to legitimize his position by traveling to Rome and seeking confirmation by Pope Julius III (1550–1555).

[16] Receiving support from the Franciscan missionaries, he arrived in Rome and entered into full communion with the Catholic Church in February 1553.

[16] Union with Rome was actively opposed by Patriarch Shemon VII Ishoyahb, who continued to reside in the Rabban Hormizd Monastery near the ancient Assyrian town of Alqosh.

[26] The Eliya line of traditionalist Patriarchs continued throughout the entire 18th century, residing in the ancient Monastery of Rabban Hormizd, which was eventually attacked and looted by muslim Turks in 1743, at the beginning of the Ottoman–Persian War (1743–1746).

[27] Faced with a centuries-old rivalry and frequent conflicts between two mighty Islamic empires (Ottoman and Persian), all Christian communities in the bordering regions were constantly exposed to danger – and not only in the times of war, since local, mainly Kurdish, warlords were accustomed to attacking Assyrian and Armenian Christian communities and monasteries and taking their land, often with Ottoman support.

[29][22] During the second half of the 16th century, traditionalist patriarchs of the Eliya line were faced with the continuous presence of the pro-Catholic movement, led by successors of Shimun VIII Yohannan Sulaqa.

[31] That interlude was ended by his successor Shimun IX Dinkha (1580–1600) who restored full communion with the Catholic Church, and was officially confirmed by the Pope in 1584.

[33] His successor Shimun XI Eshuyow (1638–1656) restored communion with the Catholic Church as late as 1653, eventually receiving confirmation from the Pope.

When the next Patriarch Shimun XII Yoalaha decided to send his profession of faith to the Pope, he was deposed by his bishops because of his pro-Catholic attitude.

Only after the death in 1827 of the last representative of the Josephite line, Joseph V Augustine Hindi, was Yohannan recognized as the Chaldean Catholic Patriarch by the Pope, in 1830.

Still based in Qodchanis, Assyrian Patriarch Shimun XVI Yohannan was not able to secure control over the traditional seat of the former Eliya line in the ancient Rabban Hormizd Monastery; and around 1808 that venerated monastic institution passed to the Chaldean Catholics.

[47] Despite the odds, the Assyrians fought successfully against the Ottomans and their allies for three years throughout southeastern Turkey, northern Iraq, northwestern Iran and northeastern Syria.

[48] Those who survived fled into Iran with what remained of the Assyrian defense under Agha Petros, but they were pursued into Iranian territory despite the fact they were fleeing.

Later, in 1918, after the murder of their de facto leader and Patriarch Shimun XIX Benyamin and 150 of his followers during a negotiation, fearing further massacres at the hands of the Turks and Kurds, most of the survivors fled by train from Iran into what was to become Iraq.

Following the end of the British mandate in 1933[48] and a massacre of Assyrian civilians at Simele by the Iraqi Army, the patriarch was forced to take refuge in Cyprus.

He transferred his residence to San Francisco in 1954, and was able to travel to Iran, Lebanon, Kuwait, and India, where he worked to strengthen the church.

[50] In 1964, the patriarch decreed a number of changes to the church, including liturgical reform, the adoption of the Gregorian calendar, and the shortening of Lent.

[52] Almost a year after the death of Shimun, Mar Khnanya Dinkha, Metropolitan of Tehran, convened a synod of seven Assyrian bishops which took place at St Paul's Abbey, Alton, in England, from 12 to 17 October 1976.

[54] On 26 March 2015, Dinkha IV died in the United States, leaving the Assyrian Church of the East in a period of sede vacante until 18 September 2015.

[60][61] On 6 September 2021, Mar Gewargis III formally stepped down as Catholicos-Patriarch during an Extraordinary Session of the Holy Synod of the Assyrian Church of the East, leaving the Patriarchal See vacant.

The Assyrian Church follows trinitarian doctrines, expressed in the Nicene Creed, and professes the eternal procession of the Holy Spirit from the Father.

The use and exact meaning of that term has been the subject of many debates, not only throughout history but also in modern times, since the Assyrian Church of the East has distinctive views on several Christological questions and claims that its theological doctrines and traditions are essentially Orthodox, while admitting the need for further inter-Christian dialogue that would resolve various questions in the field of comparative Christological terminology.

In order to preserve the impassibility of Christ's Divine Nature, the unity of his person is defined in a looser fashion than in the Alexandrian tradition.

Nestorianism has come to mean radical Dyophysitism,[68] in which Christ's two natures are eternally separate, though it is doubtful whether Nestorius ever taught such a doctrine.

[69] In the controversy that followed the Council of Ephesus, the term "Nestorian" was applied to all doctrine upholding a strictly Antiochene Christology.

In consequence, the Church of the East was labelled Nestorian, though its official Christology was in fact defined by Babai the Great, at the council that was held in 612.

[71] Iconography has been present in the Church of the East's history; opposition to religious images eventually became the norm due to the spread of Islam in the region, which forbade any type of depictions of saints and biblical prophets.

[74] Three Syriac manuscripts from the early 19th century and earlier—they were edited into a compilation titled The Book of Protection by Hermann Gollancz—contain a number of illustrations which are more or less crude.

[11] As of May 2024[update] Retired: On November 11, 1994, a historic meeting between Patriarch Dinkha IV and Pope John Paul II took place in Rome.