Islam is a religion founded by Muhammad in ancient Arabia in the 7th century. People who follow Islam are called Muslims. They believe in only one God, that God is called Allah, which is the Arabic phrase for "the (only) God". There is no plural for Allah in the Arabic language. Muslims believe that Muhammad was the last prophet (or messenger) of God. Muslims read a holy book called the Qur'an, sometimes also spelled "Koran". Muslims also look to other writings, the Sunnah and Hadith, as important guides. In Islam there is only one unforgivable sin, that is worshipping another god or giving gods qualities to a person, animal or drawing.