Islam is the official state religion in the Republic of Iraq, but the constitution guarantees freedom of religion. Iraq is a multi ethnic and multi religious country with Islam, Christianity, Yazdanism, Zoroastrianism, Shabakism, Judaism, Mandaeism, Bah?'i, Ahl-e Haqq-Yarsanis, Ishikism and numerous other religions all having a presence in the country. Shia Islam is the main religion in Iraq followed by 60-65% of the population, while Sunni Islam is followed by 32-37% of the people. Many cities throughout Iraq have been areas of historical prominence for both Shia and Sunni Muslims, including Najaf, Karbala, Baghdad and Samarra.
Iraq's Muslims follow two distinct traditions, Shia and Sunni Islam. According to the 2018 CIA Factbook, Iraq is 95% Muslim: 64-69% Shia (Arabs, Feyli Kurds et al.) and 29-34% Sunni (Arabs and Kurds). The overall percentage of Muslims has increased since the Iraqi Civil War (2014-present) due to the migration of Christian and Yezidi refugees into neighboring countries. A 2011 Pew Research Center Survey found that Iraqi Muslims are 51% Shia, 42% Sunni, and 5% described themselves as "Just a Muslim". Iraq is home to many religious sites important for both Shia and Sunni Muslims. Baghdad was a hub of Islamic learning and scholarship for centuries and served as the capital of the Abassids. The city of Karbala has substantial prominence in Shia Islam as a result of the Battle of Karbala, fought on the site of the modern city on October 10, 680. Similarly, Najaf is renowned as the site of the tomb of Al? ibn Ab? T?lib (also known as "Im?m Al?"), whom the Shia consider to be the righteous caliph and first im?m. The city is now a great center of pilgrimage from throughout the Shi'a Islamic world and it is estimated that only Mecca and Medina receive more Muslim pilgrims. The city of Kufa was home to the famed Sunni scholar Abu Hanifah, whose school of thought is followed by a sizable number of Sunni Muslims across the globe. Likewise, Samarra is also home to the al-Askari Mosque, containing the mausoleums of the Ali al-Hadi and Hasan al-Askari, the tenth and eleventh Shia Imams, respectively, as well as the shrine of Muhammad al-Mahdi, known as the "Hidden Imam", who is the twelfth and final Imam of the Shia of the Ja'far? Madhhab. This has made it an important pilgrimage centre for Ja'far? Shia Muslims. In addition, some female relatives of the Prophet Mohammad are buried in Samarra, making the city one of the most significant sites of worship for Shia and a venerated location for Sunni Muslims.
Smaller sects of Islam exist in the country, such as the small Shia Shaykhist community concentrated in Basra and Karbala. The Muslim population of Iraq is approximately 60-65 percent Shi'a, 15-20 percent Arab Sunni and 17 percent Sunni Kurdish. Iraqi Kurds are mainly secular Sunnis, with a sizeable Shia Feyli population. Most Kurds are located in the northern areas of the country, with most following the Shafi school of Islamic law. With some being members of either the Qadiri or the Naqshbandi Sufi tariqah.
Christianity was brought to Iraq in the first century by the Apostle Thomas, Addai (Thaddaeus) and his pupils Aggagi and Mari. Thomas and Thaddeus belonged to the twelve Apostles. Iraq's Chaldean minority represents roughly 3% of the population (earlier CIA Factbook), mostly living in Northern Iraq, concentrated in the Ninewa and Dahuk governorates. There are no official statistics, and estimates vary greatly. In 1950 Christians may have numbered 10-12% of the population of 5.0 million. They were 8% or 1.4 in a population of 16.3 million in 1987 and 1.5 million in 2003 of 26 million. Emigration has been high since the 1970s. Since the 2003 Iraq War, Iraqi Christians have been relocated to Syria in significant but unknown numbers. There has been no official census since 2003 the Christian population in Iraq numbers 1.2-2.1 Million.
Iraqi Christians are divided into four church bodies:
The Yazidis are an ethno-religious group in Iraq who number just over 650,000. Yazidism, or Sherfedin, dates back to pre-Islamic times. Mosul is the principal holy site of the Yazidi faith. The holiest Yazid shrine is that of Sheikh Adi located at the necropolis of Lalish.
Zoroastrianism has become the fastest growing religion with Kurds, especially in Kurdish-controlled Northern Iraq. Because of the religion's strong ties to Kurdish culture, there has been a recent rebirth of Zoroastrianism in the region, and as of August 2015 the Kurdistan Regional Government (KRG) officially recognized Zoroastrianism as a religion within Kurdish Iraq. Arguably the world's oldest monotheistic religion, Zoroastrianism (Zardashti in Kurdish) has almost disappeared in the last century until recent years. It has been estimated that as many as 100,000  to 200,000  people in Iraqi Kurdistan have converted to Zoroastrianism from a Muslim background since 2015, with the first new Zoroastrianism temples being built and opened in 2016.
Zoroastrianism was one of the dominant religions in Kurdistan before the Islamic era. Currently, Zoroastrianism is an officially recognized religion in Iraqi Kurdistan and Iran. Many Kurdish individuals converted from Islam to Zoroastrianism especially, after ISIS attacks on Kurdistan.
On 21 September 2016, the first official Zoroastrian fire temple of Iraqi Kurdistan opened in Sulaymaniyah. Attendees celebrated the occasion by lighting a ritual fire and beating the frame drum or daf.
There are no accurate numbers on the population of Zoroastrians in Iraq because they are listed as "Muslims" on their government-issued documents.
The Mandaean faith has existed in Iraq since the reign of Artabanus V of Parthia, according to the Haran Gawaitha, a text that tells the history of the Mandaean people. This would make the Iraqi presence of Mandaeans at least 1,800 years old, making it the third oldest continually-practiced faith in Iraqi society after Zoroastrianism and Judaism. There are about 60,000 estimated Mandaeians living in Iraq. The oldest independent confirmation of Mandaean existence in the region is the Kartir inscription. The Mandaean faith is commonly known as the last surviving Gnostic faith and its adherents believe it to be the oldest faith on Earth. John the Baptist, known as Yahia Yuhanna, is considered to have been the final Mandaean prophet and first true Ris'Amma, or Ethnarch, of the Mandaean people. Most Iraqi Mandaeans live near waterways because of the practice of total immersion (or baptism) in flowing water every Sunday. The highest concentrations are in the Mesene province with headquarters in Amarah, Qalat Saleh and Basra. Besides these southern regions bordering Khuzestan in Iran, large numbers of Mandaeans can be found in Baghdad in the Dweller's Quarters, giving them easy access to the Tigris River.
Judaism first came to Iraq under the rule of the Babylonian king Nebuchadnezzar II of Babylon. It was a part of the Babylonian Captivity. After the Six-Day War in Israel, rioting caused the majority of Jews to flee. Present estimates of the Jewish population in Baghdad are eight (2007), seven (2008) and five (2013). Among the American forces stationed in Iraq, there were only three Jewish chaplains.
Irreligion is thought to be rare in Iraq.