For some people, no doubt, converting to Islam was or subsequently became a matter of profound spiritual or psychological transformation, but far more often, at least initially, it was “social conversion,” motivated more by convenience than conviction.9 It happened at various rates and in different ways, but in the four centuries or so after the death of Muhammad, millions of individuals and many whole societies within the Arab Empire found their cultural identity bound up with a belief in Allah and the message of his prophet. They had become Muslims. How had this immense cultural change occurred?

In some ways, perhaps, the change was not so dramatic, as major elements of Islam—monotheism; ritual prayer and cleansing ceremonies; fasting; divine revelation; the ideas of Heaven, Hell, and final judgment—were quite familiar to Jews, Christians, and Zoroastrians. Furthermore, Islam was from the beginning associated with the sponsorship of a powerful state, quite unlike the experience of early Buddhism or Christianity. Conquest called into question the power of old gods, while the growing prestige of the Arab Empire attracted many to Allah. Although deliberately forced conversion was rare and forbidden, living in an Islamic-governed state provided a variety of incentives for claiming Muslim identity. Slaves and prisoners of war were among the early converts, particularly in Persia. Converts could also avoid the jizya, the tax imposed on non-Muslims. People aspiring to official positions found conversion to Islam an aid to social mobility. In Islam, merchants found a religion friendly to commerce. The Prophet himself had been a trader, acting as a commercial agent for his wife Khadija. As Islamic law developed over several centuries, it defined what merchants might expect from one another and so reduced the uncertainty of long-distance commerce. And in the expansive Arab Empire, merchants enjoyed a huge and secure arena for trade.

Conversion was not an automatic or easy process. Vigorous resistance delayed conversion for centuries among the Berbers of North Africa; a small group of zealous Spanish Christians in the ninth century provoked their own martyrdom by publicly insulting the Prophet; and some Persian Zoroastrians fled to avoid Muslim rule. More generally, though, a remarkable and lasting religious transformation occurred throughout the Arab Empire.

In Persia, for example, between 750 and 900, about 80 percent of the population made the transition to a Muslim religious identity. But they did so in a manner quite distinct from the people of Iraq, Syria, Egypt, and North Africa. In these regions, converts to Islam gradually abandoned their native languages, adopted Arabic, and came to see themselves as Arabs. In Iran or Persia, by contrast, Arab conquest did not involve cultural Arabization, despite some initial efforts to impose the Arabic language. By the tenth century, the vast majority of Persians had become Muslims, but the Persian language (called Farsi in Iran) flourished, enriched now by a number of Arabic loan words and written in an Arabic script. In 1010, that language received its classic literary expression when the Persian poet Ferdowsi completed his epic work, the Shahnama (The Book of Kings). A huge text of some 60,000 rhyming couplets, it recorded the mythical and pre-Islamic history of Iran and gave an enduring expression to a distinctly Persian cultural identity. Thus, in places where large-scale Arab migration had occurred, such as Egypt, North Africa, and Iraq, Arabic culture and language, as well as the religion of Islam, took hold. Such areas are today both Muslim and Arab, while the peoples of Iran, Turkey, Pakistan, Indonesia, and West Africa, for example, have “Islamized” without “Arabizing.”

The preservation of Persian language and culture had enormous implications for the world of Islam. Many religious ideas of Persian Zoroastrianism—an evil satanic power, final judgment, Heaven and Hell, Paradise—found their way into Islam, often indirectly via Jewish or Christian precedents. In Iran, Central Asia, India, and later in the Ottoman Empire, Islam was accompanied by pervasive Persian influences. Persian administrative and bureaucratic techniques; Persian court practices with their palaces, gardens, and splendid garments; Persian architecture, poetry, music, and painting—all of this decisively shaped the high culture of these eastern Islamic lands. One of the Abbasid caliphs, himself an Arab, observed: “The Persians ruled for a thousand years and did not need us Arabs even for a day. We have been ruling them for one or two centuries and cannot do without them for an hour.”10