In 610 CE, in a cave outside Mecca, a forty-year-old caravan merchant named Muhammad began to receive what he understood to be revelations from God. He preached for twenty-three years; he died in 632. Within a hundred years of his death, the religious community he founded ruled an empire stretching from Spain to the Indus, governing more land than Rome at its peak, and far more diverse a population. Nothing in human history had moved that fast.
The conquests are part of the explanation, but only part. The deeper engine was that Islam offered, to a confessionally fractured late-antique world, a startlingly clean package: a single God, a single book, a single sacred language, a body of law applicable to merchant and farmer alike, and a remarkable insistence on the equality of all believers before that law. It did not abolish slavery, nor did it grant women modern rights, but it offered a coherent moral universe that travelled well from Berber tribesman to Persian bureaucrat to Bengali peasant. The Caliphate's centuries of high civilization — Baghdad, Córdoba, Cairo, Samarkand — preserved Greek philosophy, invented algebra, advanced medicine, and held the line of the world's literacy when Latin Europe could not read its own past.
Islam is now the religion of about two billion people across more than fifty states. Many of the most consequential political fault-lines of the next decades — in the Middle East, in South Asia, in Europe — sit on top of unresolved questions about how a 7th-century revelation accommodates 21st-century pluralism. The argument is internal to the tradition, and it is not finished.