Religion in Iran has been shaped by multiple religions and sects over the course of the country's history.

Zoroastrianism was the main followed religion during the Achaemenid Empire (550–330 BC), Parthian Empire (247 BC–224 AD), and Sasanian Empire (224–651 AD). Another Iranian religion known as Manichaeanism was present in Iran during this period. Jewish and Christian communities (the Church of the East) thrived, especially in the territories of northwestern, western, and southern Iran—mainly Caucasian Albania, Asoristan, Persian Armenia, and Caucasian Iberia. A significant number of Iranian people also adhered to Buddhism in what was then eastern Iran, such as the regions of Bactria and Sogdia.

Between 632–654 AD, the Rashidun Caliphate conquered Iran, and the next two centuries of Umayyad and Abbasid rule (as well as native Iranian rule during the Iranian Intermezzo) would see Iran, although initially resistant, gradually adopt Islam as the nation's predominant faith. Sunni Islam was the predominant form of Islam before the devastating Mongol conquest (1219–1221 AD), but with the advent of the Safavid Empire (1501–1736), Shia Islam became the predominant faith in Iran.Fensham, F. Charles, "The books of Ezra and Nehemiah" (Eerdmans, 1982) p. 1

There have been a number of surveys on the current religious makeup of Iran. Those using telephone and face-to-face survey modes show a very high percentage of Iranian identifying as Muslim—99.38% in the 2011 Iranian census, and 96.6% in the World Values Survey in 2020. In 2020, the CIA World Factbook estimated that 98.5% of the population adhered to Islam. Of Iran's Muslim population, 90-95% are Shia and 5-10% are Sunni. The U.S. News & World Report placed Iran 3rd on the ranking of the most religious nations in 2024. A 2024 Gallup report conducted across multiple countries, including Iran, stated that in Iran, 72% of respondents said they were religious, 27% said they were not religious, and 1% said they were atheist.

Online surveys conducted by have reported a significantly lower percentage of Iranians adhering to Islam, with 40.4% in June 2020, 61% in February 2022, 43.5% in December 2022, and 43.8% in July 2023. The reliability of these surveys has been called into question, as they use self-selecting participants reached through social media and chain referrals.

In 2024, Iran was scored zero out of four for religious freedom by Freedom House. Christianity, Judaism and Zoroastrianism are officially recognized and protected, and have reserved seats in the Iranian parliament.Colin Brock, Lila Zia Levers. Aspects of Education in the Middle East and Africa Symposium Books Ltd, 7 mei 2007 p. 99 Iran is home to the second largest Jewish community in the Muslim world and the Middle East. The three largest non-Muslim religious minorities in Iran are the followers of the Baháʼí Faith, Christianity and Yarsanism. Sometime after 1844, the Baháʼí community became the largest religious minority group in Iran; they have been persecuted throughout their existence and are not recognized as a faith by the Iranian government.United Nations (2 November 2005) Human rights questions: human rights situations and reports of special rapporteurs and representatives General Assembly, Sixtieth session, Third Committee. A/C.3/60/L.45

History

Prehistory

The first known religions in Iran were the Elamite religions, and Indo-European religions such as Mithraism and Zoroastrianism developed from Indo-Iranian religion with slight mixture from the Elamite religion and surrounding cultures such as Assyrian, Babylonian, and Akkadian.

Zoroastrianism

The written Zoroastrian holy book, called the Avesta, dates back to between 600 and 1000 BC, but the traditions it is based on are more ancient. It was the predominant religion in the region until the Muslims conquered Iran.

Zoroastrians in Iran have had a long history reaching back thousands of years, and are the oldest religious community of Iran that has survived to the present day. Prior to the Muslim Arab invasion of Iran, Zoroastrianism had been the primary religion of Iranian peoples. Zoroastrians mainly are ethnic Persians and are concentrated in the cities of Tehran, Kerman, and Yazd. According to the Iranian census data from 2011 the number of Zoroastrians in Iran was 25,271. Reports in 2022 show a similar figure.

Oppression has led to a massive diaspora community across the world, in particular, the Parsis of India, whose numbers significantly higher than the Zoroastrians in Iran.

Mithra ( Miθra, Miça) is the Zoroastrian Divinity (yazata) of Covenant, Light, and Oath. In addition to being the divinity of contracts, Mithra is also a judicial figure, an all-seeing protector of Truth, and the guardian of cattle, the harvest, and of the Waters.

Manichaeism

Manichaeism was a major religionR. van den Broek, Wouter J. Hanegraaff Gnosis and Hermeticism from Antiquity to Modern TimesSUNY Press, 1998 p. 37 founded by the Iranian prophet Mani (Middle Persian Mānī, New Persian: Mānī, Syriac Mānī, Greek , ) in the Sasanian Empire but has been extinct for many centuries. It originated in third century Mesopotamia and spread rapidly throughout North Africa to Central Asia during the next several centuries.

Mani was a Babylonian prophet born in 216 C.E. near the city of Ctesiphon.  Not long after his birth, Mani's father, Pattikios, heard a voice commanding him to join a communitarian sect that resided in the marshes south of the city and so he abandoned his former life and took his son with him.  Mani grew up in the sect and occasionally experienced “revelations” meditated through an angelic figure.  These revelations led to his increasingly disruptive behavior and he was eventually forced to leave the sect and start a new phase of his life.

Inspired by the messages he received from the angelic figure, Mani began his missionary journeys to spread his new religion.  Gaining the favor of the Sasanian ruler in Mesopotamia was an important factor for the early success of his work.  Over time, Mani built a following and a number of his trusted disciples were dispatched to the West to Syria, Arabia, and Egypt and added more converts to this rapidly expanding religion.

Manichaeism taught an elaborate dualistic cosmology describing the struggle between a good, spiritual world of light, and an evil, material world of darkness.

By the end of the third century Manichaeism reached the attention of the Roman Empire who viewed it as a “Persian aberration” with followers who were “despicable deviants”.  Meanwhile, in Mesopotamia, rule was taken over by a new, less tolerant regime who imprisoned and executed Mani as an offender against Zoroastrian orthodoxy. As a result of consistent waves of persecution from Christians, Zoroastrians, and Muslims, Manichaeism was eventually eradicated as a formal religious affiliation within Byzantine and Islamicate realms.

Islam

Islam has been the official religion and part of the governments of Iran since the Arab conquest of Iran 640 AD. It took another few hundred years for Shia Islam to gather and become a religious and political power in Iran. In the history of Shia Islam the first Shia state was Idrisid dynasty (780–974) in Maghreb, a region of Northwest Africa. Then the Alavids dynasty (864–928 AD) became established in Mazandaran (Tabaristan), in northern Iran. The Alavids were of the Zaidiyyah Shia (sometimes called "Fiver".)Article by Sayyid 'Ali ibn 'Ali Al-Zaidi, A short History of the Yemenite Shiites (2005) Referencing: Iranian Influence on Moslem Literature These dynasties were local. But they were followed by two great and powerful dynasties: Fatimid Caliphate which formed in Ifriqiya in 909 AD and the Buyid dynasty emerged in Daylaman, in north central Iran, about 930 AD and then extended rule over central and western Iran and into Iraq until 1048 AD. The Buyid were also Zaidiyyah Shia. Later Sunni Islam came to rule from the Ghaznavids dynasty (975–1187 AD) through to the Mongol invasion and establishment of the Ilkhanate which kept Shia Islam out of power until the Mongol ruler Ghazan converted to Shia Islam in 1310 AD.

The distinction between Shia groups have distinctions between Fiver, Sevener and Twelver, derived from their belief in how many divinely ordained leaders there were who are descendants of the Islamic prophet Muhammad through his daughter Fatimah and his son-in-law Ali. These Imams are considered the best source of knowledge about the Quran and Islam, the most trusted carriers and protectors of Muḥammad's Sunnah (habit or usual practice) and the most worthy of emulation. In addition to the lineage of Imams, Twelvers have their preferred hadith collections – The Four Books – which are narrations regarded by Muslims as important tools for understanding the Quran and in matters of jurisprudence. For Twelvers the lineage of Imams are known as the Twelve Imams. Of these Imams, only one is buried in Iran – at the Imam Reza shrine, for Ali ar-Ridha who lived from 765 to 818 AD, before any Shi'a dynasties arose in Iran. The last Imam recognized by Twelvers, Muhammad al-Mahdi, was born in 868 AD as the Alavids spread their rule in Iran while in conflict with Al-Mu'tamid, the Abbasid Caliph at the time. Several Imams are buried in Iraq, as sites of pilgrimage, and the rest are in Saudi Arabia. In addition two of the Five Martyrs of Shia Islam have connections to Iran – Shahid Thani (1506–1558) lived in Iran later in life, and Qazi Nurullah Shustari (1549–1610) was born in Iran. The predominant school of theology, practice, and jurisprudence (Madh'hab) in Shia Islam is Jafari established by Ja'far as-Sadiq. There is also a community of Nizari Ismailis in Iran who recognise Aga Khan IV as their Imam.The Origins of the Sunni/Shia split in Islam by Hussein Abdulwaheed Amin, Editor of IslamForToday.com

Although Shias have lived in Iran since the earliest days of Islam, and there had been Shia dynasties in parts of Iran during the 10th and 11th centuries, according to Mortaza Motahhari the majority of Iranian scholars and masses remained Sunni until the time of the Safavids.

However, there are four high points in the history of Shia in Iran that expanded this linkage: First, the migration of a number of persons belonging to the tribe of the Ash'ari from Iraq to the city of Qum towards the end of the 7th century AD, which is the period of establishment of Imami Shi‘ism in Iran. Second, the influence of the Shia tradition of Baghdad and Najaf on Iran during the 11th to 12th centuries AD. Third, the influence of the school of Hillah on Iran during the 14th century AD. Fourth, the influence of the Shi‘ism of Jabal Amel and Bahrain on Iran during the period of establishment of the Safavid rule.