Everything changed in the Arab world in the sixth century CE. A man named Muhammad was born in 570 CE to a powerful clan of merchants, the Quraysh, who controlled various trade enterprises in Mecca and surrounding cities. He grew up to be a merchant, marrying a wealthy and intelligent widow named Khadija and traveling with caravans. He was particularly well known as a fair and perceptive arbitrator of disputes among other Arab tribes and merchants. He traveled widely on business, dealing with both Christians and Jews in Palestine and Syria, where he learned about their respective religions.
An introspective man who detested greed and corruption, Muhammad was in the habit of retreating to hills near Mecca, where there was a cave in which he would camp and meditate. When he was about forty, he returned to Mecca and reported that he had been contacted by the archangel Gabriel, who informed him that he, Muhammad, was to bear God's message to the people of Mecca and the world. The core of that message was that the one true God, the God of Abraham, venerated already by the Jews and Christians, had called the Arabs to cast aside their idols and unite in a community of worshippers.
Muhammad did not meet with much success in Mecca in his initial preaching. The temples of the many gods there were rich and powerful and people resented Muhammad's attempts to get them to convert to his new religion, in large part because he was asking them to cast aside centuries of religious tradition. The real issue with Muhammad's message was its call for exclusivity – if Muhammad had just asked the Meccans to venerate the God of Abraham in addition to their existing deities, it probably would not have incited such fierce resistance, especially from the clan leaders who dominated Meccan society. Those clan leaders were fearful that if Muhammad's message caught on, it would threaten the pilgrims who flocked to Mecca to venerate the various deities: that would be bad for business.
Thus, in 622 CE, Muhammad and a group of his followers left Mecca, exiled by the powerful families that were part of Muhammad’s own extended clan, and traveled to the city of Yathrib, which Muhammad later renamed Medina (“the city of the Prophet”), 200 miles north. They were welcomed there by the people of Medina who hoped that Muhammad could serve as an impartial mediator in the frequent disputes between clans and families. Muhammad’s trek to Medina is called the Hejira (also spelled Hijra in English) and is the starting date of the Islamic calendar.
In Medina, Muhammad met with much more success in winning converts. He quickly established a religious community with himself as the leader, one that made no distinction between religious and political authority. His followers would regularly gather to hear him recite the Koran, which means “recitations": the repeated words of God Himself as spoken to Muhammad by the angel. In 624, just two years after his arrival in Medina, Muhammad led a Muslim force against a Meccan army, and then in 630 CE, he conquered Mecca, largely by skillfully negotiating with his former enemies there – he promised to make Mecca the center of Islam, to require pilgrimage, and to incorporate it into his growing kingdom. He sent missionaries and soldiers across Arabia, as well as to foreign powers like Byzantium and Persia. By his death in 632, Muhammad had already rallied most of the Arab tribes under his leadership and most willingly converted to Islam.