The Assyrians were Christianized in the first to third centuries in Roman Syria and Roman Assyria.The population of the Sasanian province of Asōristān was a mixed one, composed of Assyrians, Arameans in the far south and the western deserts, and Persians.From the later 2nd century, the Roman Senate included several notable Assyrians, including Tiberius Claudius Pompeianus and Avidius Cassius.
At the subsequent Council of Seleucia-Ctesiphon of 410, the Christian communities of Mesopotamia renounced all subjection to Antioch and the "Western" bishops and the Bishop of Seleucia-Ctesiphon (modern al-Mada'in) assumed the rank of Catholicos.
Whereas Latin and Greek Christian cultures became protected by the Roman and Byzantine Empires, respectively, Assyrian/Syriac Christianity often found itself marginalised and persecuted.
Emigration was triggered by such events as the Assyrian Genocide in the Ottoman Empire during World War I, the Simele Massacre in Iraq in 1933, the Iranian Revolution of 1979, Arab Nationalist Ba'athist policies in Iraq and Syria such as the al-Anfal campaign of Saddam Hussein, and the rise of ISIS and its takeover of most of the Nineveh Plains.
The churches that constitute the East Syrian rite include the Assyrian Church of the East, Ancient Church of the East, and Chaldean Catholic Church, whose followers mostly speak the Northeastern branch of East Aramaic.
An advertisement from the Russian Orthodox Church that stated "There IS a God, BELIEVE.