Persia converted to Islam much more easily compared to other regions, although naturally it still took some time. The difference between Persians converting to Islam as opposed to people converting in other parts of the world was that almost all Persians converted to Islam over the course of three-four centuries. Unlike other regions, such as Spain, whose original residents remained Christians and later were able to take control over Spain again, or regions where large Christian communities continued to thrive even if they never gained control again of the region.
Bertold Spuler writes: ‘Almost all the Persians became Muslims within a few centuries without significant outside coercion on the part of their conquerors. This contrasted with the situation in Mesopotamia, Syria, Palestine, and Spain where large Christian communities remained in existence for centuries and either victoriously prevailed, as in Spain through the re-conquest undertaken by the Christian kingdoms of the North, or survived as smaller communities, as in Egypt, Syria, and Palestine.’ 
The quick adaption to Islam by the Persians can be seen referenced in many historical reports. Although Zoroastrians continued to live in many cities for over two to four centuries, Persia no longer remained their main hub by fourth-century hijri as can be ascertained by reading the work Aḥsan al-Taqāsīm (The Best Divisions for Knowledge of the Regions) by Maqdisi (d. 390). Here are a few reasons that contributed to the acceptance of Islam by the Persians over the centuries:[X]
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- The cultural and educational superiority of Islam was a lot more attractive to the Persians than Zoroastrianism. Zoroastrianism had been mixed with a Sassanid caste system and had come to present itself as a religion that caters to the nobles. It was only natural then for people to accept Islam over Zoroastrianism if there were no other barriers preventing them.
- Another reason was military, which of course makes sense in the case of Persia, although in many regions even inside Persia, Islam spread without military domination.
In other words, the very spirit of Muslim domination and victory was important for the establishment of a kind of cultural and religious superiority. The people of Gilan and Daylam who were not under Muslim military dominance at the beginning of the conquests, remained on their religion for two centuries, until a figure like Nāṣir al-Uṭrush the Zaydi was able to spread Islam among the people for cultural reasons.
In general, the political defeat of a nation prepares the ground for a change in culture and rituals, again, provided that other conditions are met.
- Another reason was political, given that the Sassanid government, which supported the Zoroastrian religion, was defeated. The Sassanid Empire was a type of theocracy and now that an attack had been carried out on the basis of a new religion, it was primarily challenging the previous religion. As it no longer had any political support, it faced challenges.
- Another reason was social, perhaps the most important of which was the settlement of the Arabs in Persia such that many Arab tribes settled in different cities and gradually spread Islam. It should be clearly said that the settlement of Arabs in Persia has been one of the most important reasons for the success of Islam in the region. This settlement was sometimes in the form of tribes, as they occupied a neighbourhood, or sometimes when an individual took up residence in a vicinity, although their impact was only in the case if they happened to have been a prominent figure such as a scholar, companion or a tābi‘ who could help establish a school of thought by settling in a city.
It is worth mentioning that Arab tribes settled in most cities of Persia. An example of this is the city of Gorgan where numerous mosques were built during the Umayyad dynasty and were named after tribes. For example, the mosque of Bajīla, situated in the neighbourhood of ‘Alī b. Zuhayr, the mosque of Muḥārib, the mosque of Quraysh, the mosque of al-Ḥamrā’ which was popularly known as the mosque of Ibn Abī Rāfi‘, the mosque of Banī Asad, the al-‘Ashīra mosque which was popularly known as Barjūb Rāh al-‘Aṭṭār, the al-Mawālī mosque, Khath‘am mosque, the Hamdān mosque, the mosque of Banī Ḍabha, the mosque of al-Azd, the mosque of Banī ‘Ijl, the mosque of Taym b. Tha‘laba, the mosque of Banī Sinān and so on.  Ya‘qūbī in his book al-Buldān reports on the spread of Arab tribes in Persian cities, and we know that in cities like Qom the Ash‘arī tribe settled and not only spread Islam, but as well as Tashayyu‘. However, no independent and detailed research has been done on this topic so far.
Of course, sometimes the natives were dissatisfied with the presence of the Arabs, but gradually they became accustomed to them. It is related that people of Sogdia would say: ‘These people have mingled with us and we have stayed with them and they have become safe from us and we have also become safe from them.’  For an individual’s perspective, we can refer to the travel of Sa‘īd b. Jubayr to Isfahan , or the stay of Wāqid in Isfahan, who was made a muezzin by Abū Mūsa al-Ash‘arī and his descendants were still muezzin until the third century.  One interesting point is that eighteen companions of the Prophet (p) had come to Isfahan. In addition, many Persians in Iraq met the companions of the Prophet (p). For example, the following reports tells us of a Persian meeting with Imam ‘Alī (a):
Dawūd b. Sulaymān al-Isfahānī said: I was with my father at the waste disposal site of Kufa when we saw an elderly bald man on a mule that was known as Duldul, and who had people’s attention towards him. So I said, ‘O father, who is this?’ He said, ‘this is the king of the Arabs, this is ‘Alī b. Abī Ṭālib.’ 
As for the spread of Islam in Bukhara and the conflicts that were created due to Arabs taking residence in its vicinity, the following details are interesting:
The inhabitants of Bukhara became Muslims, but each time after the Muslims withdrew they apostatized. Qutaybah b. Muslim converted them to Islam three times, but they [repeatedly] apostatized and became infidels. The fourth time he made war he seized the city and established Islam thereafter much difficulty. He instilled Islam in their hearts and made it difficult for them in every way. They accepted Islam in appearance but in secret worshipped idols.
Qutaybah thought it proper to order the people of Bukhara to give one-half of their homes to the Arabs so that the Arabs might mix together with them and be informed of their sentiments. Then they would be obliged to be Muslims. In this manner he made Islam prevail and imposed the religious laws on them. He built mosques and eradicated traces of unbelief and the precepts of fire worshippers. He laboured a great deal and punished everyone who broke the decrees of the religious laws. He built a grand mosque, and ordered the people to perform the Friday prayer there so that God the Exalted would reward the people of Bukhara for this good (deed) on the final judgment. 
As the Arabs began to take up residence in different parts of the city, they began constructing mosques so that they had a place to pray. Since mosques were built by replacing fire temples, eventually the population of Zoroastrians diminished. In Ṭabaqāt al-Muḥaddithīn it is reported that Mu‘āwīya had sent someone to the state of Isfahan to extinguish the fires burning in various fire temples.
The people who had settled in the city were the dominant people and the original residents were the defeated group. With all the sanctity Islam had advised Muslims to maintain towards others, at times a kind of religious discrimination would be seen. What was certain is that the Arabs wanted security and were trying to include this in their treaties. The treaty made with the people of Isfahan states:
The amount of poll-tax (jizyah) per year is as much as you can afford, which every adult must pay to the collector. It is upon you to guide the Muslims and to direct them to their destinations and to host them day and night and to give a ride to a man on foot to a place of residence. Do not dominate any Muslim, and be kind to them, and the covenant and fulfillment of the covenant are upon you, so as long as you do so, you are protected. If you alter your ways, or the other party does so and you do not surrender, then you are no longer safe, and whoever insults a Muslim will be punished, and whoever hits a Muslim, their blood will be shed. 
In the aforementioned treaty, one can see a type of discrimination against the Persians which was an attitude held by the Umayyads. It is said about the Taziyān neighbourhood of Yazd that this neighbourhood was constructed since the arrival of the Arabs.  The settlement of the Arabs in Qazvin was significant compared to many other places and in this regard, we can refer to the history of the tribes that have resided in this city, which was a significant city in the past for the Islamic world.
Migration of Persians to Iraq and Their Return
The emigration of Persians to Iraq and their return to their homeland must be taken as another reason for the growth of Islam. For example, Salmān returned to Isfahan during the time of ‘Umar b. Khaṭṭāb. Ḥammād b. Abī Sulaymān Kūfī (d. 120) was one of the captives from the Borkhar region of Isfahan, who later became a great scholar.  Waththāb the freed-slave of Ibn ‘Abbās lived with Ibn ‘Abbās for two years and then returned back to Kashan.  Nāfi‘ b. Abī Nā‘īm Muqri’ tells the people of Medina that he was originally from Isfahan, just as Ibn Abī al-Zinād the jurist would say he was originally from Hamedan. 
These people were from the first and second centuries, and a large number of them who reached high and lofty positions in the Muslim world were originally Persians. Although their original religions are not always known to us, some may have been Zoroastrians, some Christians, and perhaps some were even Jewish, and all of them had some level of influence in the early days of Islam on the Muslim society. One interesting example is the case of Ḥumrān b. Abān. Ibn Qutayba (d. 276) says regarding him:
He was from the captives of ‘Ayn al-Tamar during the caliphate of Abū Bakr. He was a Jew and his name was Ṭūwayda. ‘Uthmān bought him and emancipated him, and he became a scribe for the caliph. One time ‘Uthman got disappointed at him and sent him to Basra and made him an administrator there. When Muṣ‘ab was killed, he began to loot Basra until Khālid b. ‘Abdillah removed him from his place. When Ḥajjāj came, he forcefully took hundred-thousand dirhams from him, but with the assistance of ‘Abdul Malik the money was returned. He had married an Arab woman and his children had also married Arab women. 
Interestingly, the name of his father was Abā, which was intentionally altered to Abān in Arabic. Even the name al-Tamarī which is a reference to the captivity of this Jew in ‘Ayn al-Tamar was altered to al-Nimrī which was an Arab clan. Muṣ‘ab in the 60s of the first-century hijri had said to him: ‘O son of a Jew, you are a Nabatean unbeliever who was taken captive at ‘Ayn al-Tamar.’  The reason for mentioning this example is to show how some individuals were quickly able to attain influential positions in the early days of Islam.
The Role of Peasants
The role of peasants – who were village leaders and landowners – in the spread of Islam, which Spuler emphasizes, means that by converting to Islam they could maintain their privileges, especially in matters of property. He writes:
Everywhere the leading class joined the religion of the conquerors, and in return the conquerors allowed them to maintain their influence and even married into their families. 
In fact, both peasants and the frontier guards could maintain their statuses to some extent by becoming Muslims. When the Sassanid king was overthrown and not replaced by anyone yet, these peasants had to find a way to survive. They could keep have kept their religion, but then they would not have been able to build a close relationship with the government. By changing their religion, of course, the path to their success was smoother. When they ended up converting, so did many of their colleagues and counterparts.
It has been claimed that the Islamization of Iran took place first among the upper echelons of society, that is, among those who were the true proprietors of Iranian culture and which also maintained the old Persian heroic traditions with their chivalric idea of life.  According to Spuler, the similarities between Zoroastrianism and Islam were not ineffective in accepting Islam. Beliefs such as the creation of the world in six days, the resurrection and hell and the angels and the demons may have eased the transition.  It appears this claim is rooted in a sort of revisionism and does not have any strong evidence.
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In some cities, converts to Islam belonged to a lower class. It has been recorded that the lower class people of Bukhara converted to Islam in groups, and gradually the noble aristocrats, who were called dehqān, converted to Islam. It should be noted that the aristocrats and local rulers had bitter disputes with one another, and this greatly contributed to the presence of Muslim Arabs in the area. Peasants were originally landlords who ruled the land under their ownership as rulers. During Hārūn al-Rashīd’s visit to Khorasan, he was the guest of a farmer for four months. So Hārūn said to his minister: ‘This dehqān went out of his way to show hospitality to a guest and did not waste even a minute of our time. We should be obliged to compensate him so that we remain protected from conceit and so that we would have shown him the utmost respect for his good services.’ In any case, this gives us a glimpse as to the important role of peasants in this region and how they protected their own power and autonomy by building good ties with the Arab conquerors.
Unfortunately, in the last half of the Umayyad dynasty, in order to get more money by charging greater tax from the non-Muslims, the Umayyads would restrict the path to conversion for many people to Islam. In order to increase the monetary value of the Bayt al-Māl, the Umayyads created a period of stagnation in the number of new converts to Islam. However, Bukhara remained strongly interested in Islam and, despite being under the influence of some pressure from local rulers, did not turn away from Islam, but vigorously sought to spread it. In the last years of the Umayyad and early Abbasid rule, Bukhara became a city with an Islamic-Arab culture, and works that had previously been written in the Pahlavi language began being translated into Arabic. Planned cultural work and the construction of mosques instead of fire temples in cases where the city was forcibly conquered was a significant way to further spread the religion.
The author of the book Tārikh-e Sistān writes, ‘Rabī‘ al-Ḥārithī came to Sīstān and established good customs and forced people to gain knowledge, learn the Qurān and its exegesis. He established justice and many arrogant people became Muslims out of kindness to his character.’  In cities that were forcibly conquered by the Muslims, the fire temples would be destroyed and mosques would be built in their place. On the contrary, if a city was conquered through a truce, the fire temples would remain, except where these temples were used for pagan and Buddhist rituals. The story of Bukhara is interesting as its residents were first idol worshipers, who then became fire worshipers, and then their places of worship became mosques.
The king would go to the bazār and sit on a throne which is today the mosque of Mākh, so that people would be encouraged to buy idols. Once everyone bought themselves an idol, they would take it home. Then this same place was turned into a fire temple and on the day the market would be set up, people would gather and would visit the fire temple to worship the fire. The fire temple remained until Islam came. Once the Muslims gained power, they built a mosque there and it is from the well-known mosques of Bukhara today. 
The fire temple of Quhunduz in Nayshābūr was changed into the Masjid Jāmi‘. In Tārīkh Nayshābūr, it says that ‘Abdullah ‘Āmir conquered Nayshābūr and destroyed the fire temple of Quhunduz and built a mosque on it. In the beginning, the people of Bukhara surrendered every time in the presence of the Arabs; but as soon as the Arabs withdrew from them, they would fall back onto their previous ways. The aforesaid quotation from Narshakhī in his history work regarding the conquest of the conquering Arabs of the city is a point that holds true even for other parts of Persia and Transoxiana. It was only natural for people to react and resist the new religion, but the fact that they soon changed their minds is a point that has its own reasons. It is known that the people of Qazvin converted to Islam faster than any other city. Ḥamdullah Mustawfī in his work written around 730 hijri clarifies some aspects of the issue:
In Kitāb al-Buldān it is recorded that Qazvin was conquered by Barā’ b. ‘Āzib (d. 71) and Zayd al-Khayl Ṭā’ī during the caliphate of ‘Umar b. Khaṭṭāb. At that time the city was protected by the boundaries constructed by Shāpūrī. The people there would fight the Muslims. After the war, the Muslims sent a message that you should become a Muslim or accept the tax. They responded back with chants: We will not become Muslims nor pay the tax, return to Makkah so that we may be saved.
The Muslims closed the city upon them and pressured them into a truce while some became Muslims. Once the Muslims left, they apostatized, until another army of Muslim led by ‘Abd al-Raḥmān Ḥārithī took control over them and it was in the reign of ‘Umar that the residents of Qazvin truly converted to Islam, until they worked hard in the way of the religion, were obedient to its teachings, and reached a lofty status. 
Tax reliefs most definitely played an effective role in converting the lower class to Islam. On the other hand, it should be noted that the Umayyads also had some structures that did not allow for this relief, and rather, they would also take the jizya from new Muslims and hindered the growth of Islam for a while. This policy resulted in the formation of the Murji’a in Khorasan, a group of Muslims who believed belief (īmān) was merely confined to the two testimonies and did not accept the strictness of Ḥajjaj and the Umayyad government regarding what it meant for a person to be a believer.
Efforts of Eastern Governments
In the first and second centuries, the conquests (futūḥ) were an important factor for the expansion of the Islamic civilization and this expansion was done at the behest of the caliphate. Later on, once local governments had established themselves in Persia, they themselves continued the practice of conquests for their own economic strength and for the purpose of propagating religion. This practice continued until the time of the Ghaznavids and later on the Ottomans and the Safavids continued conquests in the west and in the Shirwan region respectively. The objective of these later conquests was presented as the expansion of Islam, which spread to many places with the earlier conquests, although it had also spread to many places without any conquests.
One of the goals of the Samanids was to fight for the spread of Islam in the far reaches of Transoxiana and Turkestan. Ismā‘īl Sāmānī, for example, attacked the city of Taraz, in the present-day location of the original city of Auliye-Ata, and suffered much. Narshakhī writes, ‘Alas, the ruler of Taraz came out and accepted Islam alongside many peasants, and Taraz was opened up. The large church was converted into the Masjid Jāmi‘ and the sermon was given in the name of the commander of the believers Mu‘taḍid Billah.’
The strength of the Samanids caused them to take an offensive position and attack various areas inhabited by the Turks of Central Asia. It was under the control of this power that Muslims gradually moved to remote Turkic areas and spread Islam among the Turks in those regions. The Ghaznavids were a lot more violent in this regard. Although their actions were not ineffective in the advancement of Islam, they did not leave a good image of Islam in India, and Islam was never able to completely conquer the hearts of the Indians, unlike what happened in Transoxiana.
The Expansion of Islam Among the Turks Through Religious Propagation
Religious propagation was one of the main tools for the growth of Islam among the Turks of Transoxiana and the residents of Cumania, from which the Seljuks emerged. Islamic propagation in Central Asia, outside the political borders of the Caliphate, achieved successes far superior to those achieved in other parts of the Islamic world.
In the late Samanid period and after that, the influence of Islam among the Turks was mostly exercised by missionaries, some of whom were Sufis, who spread Islam by migrating to those areas and influencing the Turks. Islam spread in Tabaristan and Gilan by the Alawites. There is a lot of information about this in Tārīkh-e Ṭabaristān of Ibn Isfandīyār (alive in 613) and other sources, and even Abbasid historical sources written in Baghdad acknowledge the facts about Nāṣir al-Uṭrush efforts to spread Islam.
The Gradual Process of Conversion to Islam of the Persians
A study has shown that the process of Persians becoming Muslims slowly began in the first century and gradually increased in intensity. This intensity reached its peak in the fourth century and all people converted to Islam except the Assyrians towards the West and small communities of Zoroastrians in Yazd and Shiraz. This study is based on the fact that in principle, given the names of prominent scholars in the first four centuries of Islam, Persian names are rarely seen. For a genealogical tree up to the fourth century, what we see is that the first ancestor usually has a Persian name and is eventually succeeded by Islamic names. For example, Aḥmad b. Husayn b. Rustam, where Rustam was the individual who converted to Islam and then named his son Ḥusayn.
This method was employed by Richard Bulliet in his work Conversion to Islam in the Medieval Period: An Essay in Quantitative History. According to him, he likens the growth of Islam to the growth of a silkworm and says that in the first phase, ten to fifteen percent of people converted to Islam, then forty to sixty percent, and then in the last stage the remaining ten to fifteen percent of people become Muslims. There does remain a group who do not convert to Islam at all. He determines this by looking at the names and genealogies of prominent Persians who converted to Islam as recorded in Shadharāt al-Dhahab of ʻAbd al-Ḥayy b. Aḥmad b. al-ʻImād (d. 1089) and looks at how the ancestors of many figures with Islamic names had Persian names. Although it should be mentioned that Bulliet’s conclusions are only applicable to names before fourth century, since between the fourth to sixth centuries, Persian Muslims began using Persian names for their children once again, like in the case of the Buyids where we see this excessively.
Voluntary or Forced Conversions
One of the most common topics of discussion regarding the conversion of Persians to Islam is whether they were coerced into it or did they voluntarily become Muslims. What we find in historical literature is that both of these views are correct. The most important point to consider regarding the Persians is that it was allowed for them to remain as Zoroastrians, and so accordingly, it was not possible for the conquerors to force everyone into Islam.
Another thing to note is that Muslim conquerors would be given privileges irrespective of whether converts were from the lower or upper class. However, they did not have the right to coerce people into Islam to seek these privileges. If a city was conquered by force, the conquerors would generally demolish all the fire temples, but if it was conquered through a truce, generally the fire temples would remain, but they could make a condition with the residents that they would not be allowed to construct any newer fire temples.
On the one hand, the complexity of the dynamics involved in changing one’s religion which corresponds with the many factors that existed, make it difficult to give a clear verdict on this specific question, but on the other hand show that there were different cases, and no single procedure was followed. Jamsheed Choksy in his Conflict and Cooperation: Zoroastrian Subalterns and Muslim Elites in Medieval Iranian Society claims that most people in the cities of Azerbaijan, Khuzestan and Sistan converted to Islam voluntarily as a result of the activities of religious missionaries who preached religion to families and tribes. Choksy’s book is worth reading for anyone interested in learning about the conversion of Persians from Zoroastrianism to Islam.