Eastern rite church prevalent in Iraq, Iran, and Lebanon, united with the Roman Catholic Church since 1830, and intermittently from 1551.

Christianity in Iraq and Iran dates from the late 2nd century. In the 5th century, the Church of the East embraced Nestorianism (Nestorian), a heresy that declared Christ to be man and God the son to be his divine counterpart. The church prospered and expanded into China, the steppes of Mongol Asia, and the Malabar Coast of India until the 14th century, when the Mongol leader Timur completely destroyed the Nestorian Church east of Iraq, except in India.

Union with Rome was first realized in 1551, when the elected patriarch John Sulaka went to Rome and made his profession of the Catholic faith. From this period on, those Nestorians who became Catholics were referred to as Chaldeans. Other unions were realized in 1672, 1771, and 1778, the current unbroken line of “patriarchs of Babylonia” originating in 1830. The patriarchal residence was at first in the monastery Rabbān Hormizd, then in Mosul, and finally in Baghdad. Besides the patriarchal diocese of Baghdad, there are four archdioceses (Basra, Kirkuk, Sehna, Iran—residence at Tehrān—and Urmia, to which is united the diocese of Salmas) and seven dioceses (Aleppo, Alkosh, Amadya, Akra, Beirut, Mosul, and Zakho). The Chaldeans have preserved the ancient East Syrian liturgy of Addai and Mari, which they celebrate in Syriac.