Iraq's provisional new constitution signed in March and in force until a general election, guarantees freedom of all religions. Article Seven says Islam is the official state religion "and a source of the legislation".
"This constitution respects the Islamic identity of the majority of the Iraqi population while guaranteeing complete freedom of all other religions and religious practices," it says.
The 1970 constitution adopted under the old regime guaranteed freedom of religion and prohibited any religious discrimination. It also acknowledged that the people of Iraq consisted of "two principal nationalities", Arab and Kurd, and "other nationalities", whose rights were considered legitimate.
In December 1972, the head of the ruling Baath Party identified these by decree as the Assyrians, Chaldeans and Syriacs.
The Chaldeans, whose 600,000 people represent the majority of Christians in Iraq, are an oriental rite Catholic community. The Chaldean church emerged from the Nestorian doctrine which it renounced in the 16th century while preserving its rites.
Former Deputy Prime Minister Tariq Aziz, currently in custody, is the best known of the Chaldeans.
Many Christians started leaving
Iraq in the early 1980s
The Assyrians, believed to be approximately 50,000 in number, are Christians who have remained faithful to the Nestorian doctrine.
The Nestorian church became a dissident movement in the year 431 after the Council of Ephesus. It affirms two separate personalities within Jesus Christ, namely both a human and a divine nature, and not a single personality possessing both human and divine natures as Catholicism holds.
In Iraq, there are also Catholic and Orthodox Syriacs, Catholic and Orthodox Armenians, and more recently since the time of the British mandate after the first world war, Protestants and Roman Catholics.
Many Iraqi Christians still speak Aramaic-Syriac, the language of Jesus Christ. During the 1970s, bilingual cultural magazines in Arabic and Syriac were published and radio as well as television transmitted programmes in Aramaic.
In the northern region of Kurdistan, Christians number about 150,000, mostly Chaldeans.
Christians are represented by only one minister in the interim Iraqi government to which the US-led occupation handed over power on 28 June.
Poverty and war induced many Christians to start leaving Iraq, beginning in the early 1980s. Nearly half a million have gone in the past 15 years.