Juan Cole, writing on his blog (Dec. 25, 2003):

For the history of Iraqi Christianity click here. Iraqis believe Christianity was brought to what is now Iraq, an Aramaic-speaking area, around 35 AD by Thomas the doubting apostle (some say Peter also preached in Mesopotamia). The religions of Iraqis at that time included Babylonian-style polytheism and star worship (including astrology), Zoroastrianism from Iran, Greek Gnosticism and Judaism. In the theological disputes that developed from the 400s, most Iraqi Christians are believed by historians to have favored the Nestorian branch of Christianity, founded by Nestorius (d. 451). By the time of the Muslim Arab conquest of Iraq in the 600s AD, what is now Iraq had a significant Christian population. Over time most Iraqis gradually converted to Islam and adopted Arabic, and contrary to popular Western belief, the conversion was for the most part peaceful. From the 1400s some Iraqi Nestorians accepted overtures from Rome and acknowledged the pope, becoming Catholics. They were allowed to keep their Aramaic liturgy. These Catholic “Uniate” Iraqis became known as Chaldeans, and had their own patriarch. Over time they became the majority (now 80%). Those who remained outside Catholicism may not be exactly identified as Nestorians any more by this period, but had historical roots in that branch of Christianity, and were called Assyrians. In recent decades there has been a push to unify the Chaldeans and the Assyrians. Iraqi Christians probably amount to between 500,000 and 800,000 individuals, about 2 or 3 percent of Iraqis.