Answer:
Believing that God had chosen him as his messenger Muhammad began to preach what God had revealed to him. The simple and clear-cut message of Islam, that there is no God but Allah, and that life should be lived in complete submission to the will of Allah, was attractive to many people, and they flocked to hear it.
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All able Muslims must make a pilgrimage (hajj) to Mecca at least once in their lifetimes.
