The continuing developments in Islam since Wismar's time make this book as relevant today as it was when it was published. The question explored here is what the approach of Islam is towards religious tolerance. To get at the answer, Wismar went back to the Quran and the practice of Muhammad, Abu Bakr, and Umar Ibn al-Khattab, the first two Caliphs, in order to determine what the primary sources taught on the issue.