The word Islam, which when translated from Arabic, means "to submit to the will of Allah," is the youngest of the world's major religions. Worshippers of this monotheistic religion are known as Muslims, which means "one who submits to the will of Allah." The Islamic holy book is called the Quran. Islam is currently the second most practiced religion in the world, and experts predict that it will overtake Christianity as the most popular religion in the world sometime during the 21st century.