After Muhammad's death, Jews and Christians were never required to convert to Islam but were allowed to practise their own religion freely in the Islamic empire. Later, Zoroastrians, Hindus, Buddhists and Sikhs were also counted among the People of the Book. It has never been a problem for Muslims to coexist with people of other religions. The Islamic empire was able to pay host to Christians and Jews for centuries; but Western Europe has found it almost impossible to tolerate Muslims and Jews in Christian territory.
— Karen Armstrong, Muhammad: A Biography of the Prophet