From Karen Armstrong’s A History of God: The 4,000-Year Quest of Judaism, Christianity and Islam:

Nobody in the new empire was forced to accept the Islamic faith; indeed, for a century after Muhammad’s death, conversion was not encouraged and, in about 700, was actually forbidden by law: Muslims believed that Islam was for the Arabs as Judaism was for the sons of Jacob.  As the “people of the book,” … Jews and Christians were granted religious liberty as … protected minority groups.  When the Abbasid caliphs began to encourage conversion, many of the Semitic and Aryan peoples in their empire were eager to accept the new religion.