One of the noteworthy aspects of the Arab conquests is the complex role of conversion. The Koran specifically forbids the forcible conversion of Jews and Christians. It does allow that non-Muslim monotheists pay a special tax, however. For the century of Umayyad rule, only about 10% of the population was Muslim. Non-Muslims, called dhimmis (followers of religions tolerated by law) had to pay a head tax and were not allowed to share in governmental decision-making or in the spoils of war. Many Jews and Christians found Arab rule preferable to Byzantine rule, however, because the Byzantine government had actively persecuted religious dissenters and the Arabs did not. Likewise, taxes were lower under the Arabs as compared to Byzantium.
These traditions of relative tolerance would continue all the way up to the modern era in places like the Ottoman Empire. However, even without forcible pressure, many people did convert to Islam either out of a heartfelt attraction to Islam or because of simple pragmatism; in some cases, Muslim generals rejected the attempted conversions of local people because it threatened their tax base so much.
There was also the case of the nomadic peoples of North Africa, collectively referred to as “Berbers” by the Arabs. The Berbers were hardy, warlike tribesmen living in rugged mountainous regions across North Africa. They had already seen the Romans and the Vandals come and go and simply kept up their traditions with the arrival of the Arabs. They were, however, polytheists, which the Muslims were unwilling to tolerate. Thus, faced with the choice of forcible conversion or death, the Berbers converted and then promptly joined the Arab armies as auxiliaries. This lent tremendous strength to the Arab forces and helps explain the relative ease of their conquests, especially in Spain.
The members of other monotheistic faiths who chose not to convert were often left much more free to practice their religions than they would have been in Christian lands, because the Umayyads simply did not care about theological disagreements among their Jewish and Christian subjects so long as the taxes were paid. Over time, various sects of Christianity survived in Muslim lands that vanished in kingdoms that were officially, and rigidly, Christian. Likewise, Jews found that they were generally better off in Muslim lands than in Christian kingdoms because of their safety from official persecution. Jews became vitally important merchants, scholars, bankers, and traders all across the caliphate.
Zoroastrianism, however, declined. Muslims were less tolerant of Zoroastrianism because it did not venerate the God of Abraham and its traditions were markedly different from those of Judaism and Christianity. By the tenth century, most Zoroastrians who had not converted migrated to India, where they remain today in communities known as the Parsees.