Islam is a religion based on the teachings of Muhammad, a man from the 7th century Arabia, who claimed to receive messages from Allah (god) through angel Gabriel. People who believe it are called Muslims and there are 1.5+ billion Muslims, which makes Islam the second most wide-spread religion after Christianity. The holiest text in Islam is the Quran, which contains Muhammad's revelations. After Muhammad received the first revelation, he kept receiving more and more revelations until his death. The completion of the Quran took over twenty years and it was compiled into a book form many years after Muhammad's death. A great portion of the Quran deals with Muhammad's personal life and the early Muslim community, but Muslims believe that the Quran is the final message from the creator of the universe and people should follow it, even in the 21st century.
A great emphasis is on a so called "tawheed", which means worshiping only Allah and no other gods . Muslims are repeating phrases like "No god, but Allah!" (la ilaha illa allah) or "Allah is the greatest!" (Allahu Akbar) to emphasize the rejection of other gods. Polytheism is not very popular today, but to understand why Islam is so obsessed with it, we have to realize Muhammad was born in Mecca, where polytheists lived. Polytheism is called shirk and Islam accuses even other monotheistic religions, like Christianity of committing shirk (claiming Jesus is the son of God is a type of shirk). Although Muslims are supposed to worship god and nobody else, the only way they know Allah is from Muhammad and also Muhammad revealed that he is a perfect human being, who should be emulated and that Allah wants people to obey Muhammad, so practically, in Islam, you have to do everything Muhammad says, regardless if it was his opinion or a revelation. Muhammad's way of life is called the sunnah and it is also a part of Islam, besides the Quran. We can read about sunnah in various collections of hadith (narrations from Muhammad and his followers) and the sira (his life story).
Islam is considered to be one of Abrahamic religions, because the Quran borrows a lot from the Bible. It talks about Abraham, Moses and Jesus (Isa). Jesus is considered to be only a prophet. The Quran also contains a lot of Jewish, Christian and pagan myths which were wide-spread in 7th century Arabia.
Besides the theology and myths, Muhammad revealed a lot of verses dealing with his personal life and the early Muslim community. Muslims all over the world today, read verses about the creator of the universe being angry at Muhammad's uncle Abu Lahab. There are also a lot of commands for the Muslims. For example, Muhammad was waging wars against other Arab tribes and was preaching his revelations, which say that God wants Muhammad's army to kill and enslave people. Islam is famous among non-Muslims for being spread by the sword and for oppression of women.
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