Islam is often described as a religion of practice rather than of creed, and the description is fair as far as it goes. What must be believed is short: that God is one, without partner, offspring or image; that he has sent messengers to every nation; that scriptures were given to some of them; that angels exist; that there will be a resurrection and a judgement; and that what happens is subject to God's decree.
The Quran is understood not as a book about revelation but as revelation itself, God's speech in Arabic, received by Muhammad over some twenty-three years and unaltered since. That belief explains the weight placed on the Arabic text, on recitation as a discipline in its own right, and on the reluctance to call a translation a Quran at all. Alongside it stands the hadith, the reports of what Muhammad said, did and permitted, collected and sifted for reliability in the centuries after his death and second only to the Quran as a source of law.
Obligation is organised around five pillars: the declaration that there is no god but God and that Muhammad is his messenger; prayer five times daily facing Mecca; the alms levy; the daylight fast of Ramadan; and the Hajj, the pilgrimage to Mecca required once of those able to make it. None of this requires a priesthood. There are scholars, judges and prayer leaders, and their authority rests on learning and community recognition rather than on ordination.
Around the pillars grew sharia, less a fixed code than the body of reasoning by which scholars derive God's requirements from the sources, covering worship, diet, commerce, marriage and inheritance. Its interpretation has never been uniform, and the question of who is entitled to interpret it is among the most contested in the modern Muslim world.