Before Muḥammad preached Islam—submission to the will of Allah—the Arabs were organized in tribes, which were divided into clans or families. The desert with its oases does not permit a social and political organization larger than the family, the clan or at most the tribe. To achieve a larger and higher unity, a force is needed which is capable of binding together men of differing customs and traditions. The message of the prophet, gathered in the Qurʾān, provided this cohesion and, with the more material inducements of booty and landed property as the result of the holy war (jihād) on behalf of Allah, succeeded in winning the allegiance to Islam of independent, proud, born warriors. Muḥammad's success was such that under his successors the community of believers was extended far beyond the Arabian peninsula and finally consolidated in the vast Islamic empire in Asia, Africa, Spain and Sicily.

How was it that the message claimed by the messenger of Allah to be God's final revelation to mankind proved acceptable not only to pagans, but also to Christians, Jews and Zoroastrians—all three possessors of a book of revelation, ahl al-kitāb—and to Buddhists?