Iraqi Christians probably number about 800,000, although the country's recent turbulent history has made numbers difficult to assess, especially as the Christian heartland in Iraq has traditionally been in the north, now in semi-anarchy as rival Kurdish factions vie for control of the area.
For ecclesiastical historians, the Christian community is almost like a living museum, as it was split into different sects in schisms which date right back tothe first days of the Church.
So in 431 AD, the community was split apart by doctrinal differences over the nature of Christ. Some communities agreed with Nestorius, then the Patriarch of Constantinople, and therefore became known as Nestorian Christians, while others leapfrogged over the Eastern Orthodox churches to accept the supremacy of Rome.
So most Christians today are Catholics 'of the Chaldaean rite', accepted as Catholics by the Vatican in the same way as Maronite Christians have been in Lebanon. There are maybe 600,000 Catholics in Iraq.
But another 200,000 belong to various Orthodox churches, including 150,000 who have remained 'Nestorian Christians' to this day.
There is also a very small community of Protestants - perhaps 5,000. As elsewhere in the Middle East, Iraqi Protestants were former Catholic or Orthodox Christians converted by Western missionaries in the last two centuries after they found that they could not make any inroads into converting the Muslim majority in the country.
The terms 'Chaldaean' and 'Assyrian' are often used interchangeanlt to describe Iraq's Christians.