Islam was founded by Mohammed in what is now Saudi Arabia. At the time of Mohammed's death in 632, much of the Arabian Peninsula was already under Muslim rule. By 661, a mere three decades after his death, the religion had spread far beyond Arabia and included both Egypt and Persia. The speed of Islam's growth was extraordinary—and much different from that of Christianity and Buddhism. Much of the credit for Islam's growth is correctly attributed to the Muslims' military prowess. For instance, in 636, the Muslims crushed a formidable Byzantine army at the Battle of Yarmouk. The Persians were also unable to stop the Islamic armies. However, it would be too simplistic to attribute Islam's growth solely to the success of its warriors, and Islamic conquerors typically tolerated other religions.

One leader responsible for the growth of Islam was Ashoka the Great. Ashoka renounced war and embraced ahimsa, or non-violence. By 750, Islam had reached both modern-day Spain and Pakistan.

In contrast, Buddhism and Christianity grew much more gradually. Christians were persecuted for much of their early history. Finally, by the fourth century, Christianity was accepted when Constantine the Great became the first Christian Emperor of Rome.

The Muslims laid a foundation for the European Renaissance. For example, Aristotle's works were kept by Islamic scholars, and would have been lost to history otherwise. They also made advances in medicine, mathematics, astronomy, and geography. At the time of the Crusades, the Middle East was culturally and intellectually more advanced than Europe.