Islam, one of the three major world religions, along with Judaism and Christianity, that profess monotheism, or the belief in a single God. In the Arabic language, the word Islam means “surrender” or “submission”—submission to the will of God. A follower of Islam is called a Muslim, which in Arabic means “one who surrenders to God.” The Arabic name for God, Allah, refers to the same God worshiped by Jews and Christians. Islam's central teaching is that there is only one all-powerful, all-knowing God, and this God created the universe. This rigorous monotheism, as well as the Islamic teaching that all Muslims are equal before God, provides the basis for a collective sense of loyalty to God that transcends class, race, nationality, and even differences in religious practice. Thus, all Muslims belong to one community, the umma, irrespective of their ethnic or national background.

Within two centuries after its rise in the 7th century, Islam spread from its original home in Arabia into Syria, Egypt, North Africa, and Spain to the west, and into Persia, India, and, by the end of the 10th century, beyond to the east. In the following centuries, Islam also spread into Anatolia and the Balkans to the north, and sub-Saharan Africa to the south. The Muslim community comprises about 1 billion followers on all five continents, and Islam is the fastest-growing religion in the world. The most populous Muslim country is Indonesia, followed by Pakistan and Bangladesh. Beyond the Middle East, large numbers of Muslims live in India, Nigeria, the former republics of the Union of Soviet Socialist Republics (USSR), and China.

One of the reasons for the growth of the Muslim community has been its openness to new members. Children born to Muslim parents are automatically considered Muslim. At any time, a non-Muslim can convert to Islam by declaring himself or herself to be a Muslim. A person's declaration of faith is sufficient evidence of conversion to Islam and need not be confirmed by others or by religious authorities.

Around the year AD 570 Muhammad, the founding prophet of Islam, was born in Mecca, at the time the central city of the Arabian Peninsula. Some 40 years later Muhammad started preaching a new religion, Islam, which constituted a marked break from existing moral and social codes in Arabia. The new religion of Islam taught that there was one God, and that Muhammad was the last in a series of prophets and messengers. Through his messengers God had sent various codes, or systems of laws for living, culminating in the Qur'an (Koran), the holy book of Islam. These messengers were mortal men, and they included among many others Moses, the Hebrew prophet and lawgiver, and Jesus, whom Christians believe to be the son of God rather than a prophet.

Islam also taught that the Christian Bible (which includes the Hebrew Bible as the Old Testament and an additional 27 books referred to as the New Testament), and the Qur'an were all holy books. According to the Qur'an, the two earlier Scriptures had been altered over time from their original forms given by God, while the Qur'an would remain perfect, preserved by God from such distortion. In addition to distinguishing itself from the Hebrew and Christian traditions, the new religion taught that the God of Islam had provided humanity with the means to know good from evil, through the prophets and the Qur'an. Therefore, on the Day of Judgment people will be held accountable for their actions.

Muhammad's teachings met with severe and hostile opposition, and in the year 622 he left Mecca and sought refuge in the city of Yathrib, as a number of his followers had already done. Upon Muhammad's arrival, the name Yathrib was changed to Medina (meaning “the city”). The date of Muhammad's immigration was later set as the beginning of the 12-month lunar Islamic calendar