The official religion of Iraq is Islam, practiced by about 97% of the population. Muslims in Iraq practise two types of Islam: Shi'ism, practised by about 60% of the population, and Sunni. The rest of the population is made up mostly of Christians, Jews and adherents of others faiths.

Islam arose in the Middle Eastern city of Mecca in the 7th century, when Mohammed proclaimed himself the last in a line of prophets that includes Moses, Abraham and Jesus. Mohammed's teachings were compiled into the Islamic holy book, the Koran (or Qur'an). His other sayings and teachings, which were recorded by those who knew him, became known as the hadith (tradition). Muslims accept parts of the Biblical Old and New Testaments as indirect words from Allah (God).

The split into Sunni and Shi'ite Muslims occurred because of a disagreement over the question of religious leadership. One of the major distinctions between the two branches is that Shi'ites depend upon an imam or religious leader who, like the Pope, is considered an intermediary between the faithful and Allah. For Sunni Muslims, an imam is one who leads others in prayer.

All Muslims practice the five pillars of Islamic faith: professing the faith; praying five times daily; giving taxes for the upkeep of the poor and religious scholars; fasting during the month of Ramadan; and, if possible, making a hajj (pilgrimage) to the holy city of Mecca once in a lifetime. Shi'ites also add two more pillars: supporting the jihad or crusade to protect Islamic beliefs; and doing good works and avoiding all evil.

Muslims worship in mosques, where men congregate particularly on Friday, the Muslim Holy Day. Women are discouraged, but not prevented, from joining men at prayers in the mosque. There are also many important Islamic pilgrimage sites in Iraq, especially in the cities of An Najaf, Karbala, Kazimayn and Samarra, which contain tombs of Muslim Imams.

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Over one million Assyrians, a Semitic people indigenous to ancient Mesopotamia, still live in Iraq and form the country's largest Christian minority. The Assyrian Church was founded in 33 AD.

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Built in the 9th century, the great Mosque of Samarra has a distinctive spiral minaret. People can walk up spiralling stairs to its top.