The Islamic conquest of Persia (637-651) led to the end of the Sassanid Empire and the eventual decline of the Zoroastrian religion in Persia. However, the achievements of the previous Persian civilizations were not lost, but were to a great extent absorbed by the new Islamic polity. Islam has been the official religion of Iran since then, except short duration after Mongol raid and establishment of Ilkhanate. Iran became an Islamic republic in 1979 after Islamic Republic of Iran on the basis of its constitution.
Before the Islamic conquest, the Persians had been mainly Zoroastrian, however, there were also large and thriving Christian and Jewish communities. There was a slow but steady movement of the population toward Islam, When Islam was introduced to Iranians. The nobility and city-dwellers were the first to convert, most likely to preserve the economic and social status and advantages; Islam spread more slowly among the peasantry and the dihqans, or landed gentry. By the late 11th century, the majority of Persians had become Muslim, at least nominally. From the beginning most Persian Muslims were Sunni, although much of the nobility and most of the educated class were Shi'a Muslim, including Avicenna, Al-Farabi, Al-Biruni and Nasīr al-Dīn al-Tūsī, as well as Ferdowsi, the author of Iran's national epic Shahnameh. The authors of the Shi'a Four Books were Iranian as well. Though Iran is known today as a stronghold of the Shi'a Muslim faith, it did not become so until much later around the 15th century. The Safavid dynasty made Shi'a Islam the official state religion in the early sixteenth century and aggressively proselytized on its behalf. It is also believed that by the mid-seventeenth century most people in Iran had become Shi'as, an affiliation that has continued. Over the following centuries, with the state-fostered rise of a Persian-based Shi'ite clergy, a synthesis was formed between Persian culture and Shi'ite Islam that marked each indelibly with the tincture of the other.
The Iranian Muslims projected many of their own Persian moral and ethical values that predates Islam into the religion, while recognizing Islam as their religion and the prophet's son in law, Ali as an enduring symbol of justice.
Nowadays Islam is the religion of 98% of Iranians, but unlike the majority of the Islamic world the proportion of Shi'ah Muslims in Iran is higher than the proportion of Sunni Muslims. Approximately 89% of Iranians are Shi'a and 9% are Sunni, mostly Turkomen, a minority of Arabs (mainly in Hormozgan Province), Baluchs, and Kurds living in the south, southeast, northeast and northwest. Almost all of Iranian Shi'as are Twelvers.
Muslims invaded Iran in the time of Umar (637) and conquered it after several great battles. Yazdegerd III fled from one district to another until a local miller killed him for his purse at Merv in 651. By 674, Muslims had conquered Greater Khorasan (which included modern Iranian Khorasan province and modern Afghanistan, Transoxania, and Pakistan).
As Bernard Lewis has quoted
"These events have been variously seen in Iran: by some as a blessing, the advent of the true faith, the end of the age of ignorance and heathenism; by others as a humiliating national defeat, the conquest and subjugation of the country by foreign invaders. Both perceptions are of course valid, depending on one's angle of vision."
Under Umar and his immediate successors, the Arab conquerors attempted to maintain their political and cultural cohesion despite the attractions of the civilizations they had conquered. The Arabs were to settle in the garrison towns rather than on scattered estates. They were not to marry non-Arabs, or learn their language, or read their literature. The new non-Muslim subjects, or dhimmi, were to pay a special tax, the jizya or poll tax, which was calculated per individual at varying rates for able bodied men of military age.
In the 7th century AD, when many non-Arabs such as Persians entered Islam were recognized as Mawali and treated as second class citizens by the ruling Arab elite, until the end of the Umayyad dynasty. During this era Islam was initially associated with the ethnic identity of the Arab and required formal association with an Arab tribe and the adoption of the client status of mawali. There are a number of historians who see the rule of the Umayyads as setting up the "dhimmah" to increase taxes from the dhimmis to benefit the Arab Muslim community financially and by discouraging conversion. Mass conversions were neither desired nor allowed, at least in the first few centuries of Arab rule
Richard Bulliet's "conversion curve" indicates that only about 10% of Iran converted to Islam during the relatively Arab-centric Umayyad period. Beginning in the Abassid period, with its mix of Persian as well as Arab rulers, the Muslim percentage of the population rose. As Persian muslims consolidated their rule of the country, the Muslim population rose from approx. 40% in the mid 9th century to close to 100% by the end of 11th century. Seyyed Hossein Nasr suggests that the rapid increase in conversion was aided by the Persian nationality of the rulers.
According to Bernard Lewis:
"Iran was indeed Islamized, but it was not Arabized. Persians remained Persians. And after an interval of silence, Iran reemerged as a separate, different and distinctive element within Islam, eventually adding a new element even to Islam itself. Culturally, politically, and most remarkable of all even religiously, the Iranian contribution to this new Islamic civilization is of immense importance. The work of Iranians can be seen in every field of cultural endeavor, including Arabic poetry, to which poets of Iranian origin composing their poems in Arabic made a very significant contribution. In a sense, Iranian Islam is a second advent of Islam itself, a new Islam sometimes referred to as Islam-i Ajam. It was this Persian Islam, rather than the original Arab Islam, that was brought to new areas and new peoples: to the Turks, first in Central Asia and then in the Middle East in the country which came to be called Turkey, and of course to India. The Ottoman Turks brought a form of Iranian civilization to the walls of Vienna... "
As the power of the Abbasid caliphs diminished, a series of dynasties rose in various parts of Iran, some with considerable influence and power. Among the most important of these overlapping dynasties were the Tahirids in Khorasan (820-72); the Saffarids in Sistan (867-903); and the Samanids (875-1005), originally at Bokhara. The Samanids eventually ruled an area from central Iran to Pakistan. By the early 10th century, the Abbasids almost lost control to the growing Persian faction known as the Buwayhid dynasty(934-1055). Since much of the Abbasid administration had been Persian anyway, the Buwayhid, who were Zaidi Shia, were quietly able to assume real power in Baghdad.
The Samanid dynasty was the first fully native dynasty to rule Iran since the Muslim conquest, and led the revival of Persian culture. The first important Persian poet after the arrival of Islam, Rudaki, was born during this era and was praised by Samanid kings. The Samanids also revived many ancient Persian festivals. Their successor, the Ghaznawids, who were of non-Iranian Turkic origin, also became instrumental in the revival of Persian.
A serious internal threat to the Seljuks during their reign came from the Ismailis, Nazari Ismaili sect, with headquarters at Alamut between Rasht and Tehran. They controlled the immediate area for more than 150 years and sporadically sent out adherents to strengthen their rule by murdering important officials. Several of the various theories on the etymology of the word assassin derive from these killers.
Although Shi'as have lived in Iran since the earliest days of Islam, and there was one Shi'a dynasty in part of Iran during the tenth and eleventh centuries, but according to Mortaza Motahhari the majority of Iranian scholars and masses remained Sunni till the time of the Safavids.
However it doesn't mean Shia was rootless in Iran. The writers of The Four Books of Shia were Iranian as well as many other great Shia scholars.
Muhaqqiq Hilli mentions the names of the great Islamic jurists which most of them were Iranian.:
In view of the fact that we have a great number of Fuqaha(Islamic jurists) who have copiously written on the subject, it is not possible for me to quote all of them. I have selected from those who were best known for their research and scholarship, quoting their Ijtihad, and the opinions they adopted for action. From amongst the earlier ones, I have selected Hasan ibn Mahboob, Ahmed ibn Abi Nasr Bezanti, Husain ibn Saeed Ahvazi, Fadhl ibn Shadhan Nisaburi, Yunus ibn Abd alRahman. They lived during the presence of our Imams. From the later group, I quote Muhammad ibn Babawayh Qummi and Muhammad ibn Yaqoob Kulaini. As for the people of Fatwa, I consider the verdicts of Askafi, Ibn Abi Aqeel, Shaykh Mufid, Seyyid Murtadha Alamul Huda and Shaykh Tusi.
The domination of the Sunni creed during the first nine Islamic centuries characterizes the religious history of Iran during this period. There were however some exceptions to this general domination which emerged in the form of the Zaydīs of Tabaristan, the Buwayhid, the rule of Sultan Muhammad Khudabandah (r. Shawwal 703-Shawwal 716/1304-1316) and the Sarbedaran. Nevertheless, apart from this domination there existed, firstly, throughout these nine centuries, Shia inclinations among many Sunnis of this land and, secondly, original Imami Shiism as well as Zaydī Shiism had prevalence in some parts of Iran. During this period, Shia in Iran were nourished from Kufah, Baghdad and later from Najaf and Hillah.
However, during the first nine centuries there are four high points in the history of this linkage:
After Ismail I captured Tabriz in 1501 and established Safavids dynasty, proclaimed Twelver Shiʿism as the state religion, ordering conversion of the Sunnis. "a search of all Islamic libraries unearthed only one book on Shi'ism." Ismail brought Arab Shia clerics from Bahrain, Iraq, Syria, and Lebanon in order to preach the Shi'a faith. Isma'il's attempt to spread Shi'ite propaganda among the Turkmen tribes of eastern Anatolia prompted a conflict with the Sunnite Ottoman Empire. Following Iran's defeat by the Ottomans at the Battle of Chaldiran, Safavid expansion slowed, and a process of consolidation began in which Isma'il sought to quell the more extreme expressions of faith among his followers.
While Ismail I declared shiism as the official state religion, it was in fact his successor, Tahmasb, who consolidated the Safavid rule and spread Shiʿism in Iran. After a period of indulgence in wine and the pleasures of the harem, he turned pious and parsimonious, observing all the Shiʿite rites and enforcing them as far as possible on his entourage and subjects. Under Abbas I, Iran prospered. The monarch continued the policy begun under his predecessors of eradicating the old Sufi bands and ghulat extremists whose support had been crucial in building the state. Abbas instituted the practice of immuring infant princes in palace gardens away from the promptings of intrigue and the world at large. As a result, his successors tended to be indecisive men, easily dominated by powerful dignitaries among the Shi'ite 'ulama'—whom the shahs themselves had urged to move in large numbers from the shrine cities of Iraq in an attempt to bolster Safavid legitimacy as an orthodox Shi'ite dynasty. Succeeding Safavid rulers promoted Shi'ism among the elites, and it was only under Mullah Allamah al-Majlis - court cleric from 1680 until 1698 - that Shi'ism truly took hold among the masses.
As in the case of the early Sunnite caliphate, Safavid rule had been based originally on both political and religious legitimacy, with the shah being both king and divine representative. With the later erosion of Safavid central political authority in the mid-17th century, the power of the Shia scholars in civil affairs such as judges, administrators, and court functionaries, began to grow, in a way unprecedented in Shi'ite history. Likewise, the ulama began to take a more active role in agitating against Sufism and other forms of popular religion, which remained strong in Iran, and in enforcing a more scholarly type of Shi'ism among the masses. The development of the ta'ziah—a passion play commemorating the martyrdom of Imam Husayn and his family — and Ziarat of the shrines and tombs of local Shi'ite leaders began during this period, largely at the prompting of the Shi'ite clergy. According to Mortaza Motahhari, the majority of Iranians turned to Shi'ism from the Safavid period onwards. Of course, it cannot be denied that Iran's environment was more favorable to the flourishing of the Shi'ism as compared to all other parts of the Muslim world. Shi'ism did not penetrate any land to the extent that it gradually could in Iran. With the passage of time, Iranians' readiness to practise Shi'ism grew day by day. Had Shi`ism not been deeply rooted in the Iranian spirit, the Safawids (907-1145/ 1501-1732) would not have succeeded in converting Iranians to the Shi'i creed and making them follow the Prophet's Ahl al-Bayt sheerly by capturing political power.
In conclusion, it was the Safavids who made Iran the spiritual bastion of Shi’ism against the onslaughts of orthodox Sunni Islam, and the repository of Persian cultural traditions and self-awareness of Iranianhood, acting as a bridge to modern Iran. According to Professor Roger Savory:
In Number of ways the Safavids affected the development of the modern Iranian state: first, they ensured the continuance of various ancient and traditional Persian institutions, and transmitted these in a strengthened, or more 'national', form; second, by imposing Ithna 'Ashari Shi'ism on Iran as the official religion of the Safavid state, they enhanced the power of mujtahids. The Safavids thus set in train a struggle for power between the urban and the crown that is to say, between the proponents of secular government and the proponents of a theoretic government; third, they laid the foundation of alliance between the religious classes ('Ulama') and the bazaar which played an important role both in the Persian Constitutional Revolution of 1905–1906, and again in the Islamic Revolution of 1979; fourth the policies introduced by Shah Abbas I conduced to a more centralized administrative system.
According to scholar Roy Mottahedeh, one significant change to Islam in Iran during the first half of the 20th century was that the class of ulema lost its informality that allowed it to include everyone from the highly trained jurist to the "shopkeeper who spent one afternoon a week memorizing and transmitting a few traditions." Laws by Reza Shah that requiring military service and dress in European-style clothes for Iranians, gave talebeh and mullahs exemptions, but only if they passed specific examinations proving their learnedness, thus excluding less educated clerics.
In addition Islamic Madrasah schools became more like `professional` schools, leaving broader education to secular government schools and sticking to Islamic learning. "Ptolemaic astronomy, Aveicennian medicines, and the algebra of Omar Kahayyam" was dispensed with.
The Iranian Revolution (also known as the Islamic Revolution, Persian: انقلاب اسلامی, Enghelābe Eslāmi) was the revolution that transformed Iran from a monarchy under Shah Mohammad Reza Pahlavi to an Islamic republic under Ayatollah Ruhollah Khomeini, the leader of the revolution and founder of the Islamic Republic. It has been called "the third great revolution in history," following the French and Bolshevik revolutions, and an event that "made Islamic fundamentalism a political force ... from Morocco to Malaysia.
Sunni Muslims constitute approximately 9% of the Iranian population. A majority of Kurds, virtually all Baluchis and Turkomans, and a minority of Arabs are Sunnis, as are small communities of Persians in southern Iran and Khorasan. Shia clergy tend to view missionary work among Sunnis to convert them to Shi'ism as a worthwhile religious endeavor.. Since the Sunnis generally live in the border regions of the country, there has been no occasion for Shia-Sunni conflict in most of Iran. In those towns with mixed populations in West Azarbaijan, the Persian Gulf region, and Sistan and Baluchistan, tensions between Shi'as and Sunnis existed both before and after the Revolution. Religious tensions have been highest during major Shi'a observances, especially Moharram.