In 610 CE, in a cave on Mount Hira 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 against the entrenched polytheism of his own tribe, the Quraysh; he was driven from Mecca to Medina in 622 — the hijra that starts the Muslim calendar — and returned in triumph eight years later. He died in 632, having unified most of the Arabian Peninsula. Within a hundred years of his death, the religious community he founded ruled an empire stretching from the Atlantic coast of Spain to the Indus, governing more land than Rome at its peak, and a far more diverse population. The decisive battles came fast: the Sasanian Persian Empire collapsed entirely by 651, and Byzantium lost Syria, Egypt, and North Africa within a generation. 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 arrived, crucially, when its two great rivals — Byzantium and Sasanian Persia — had exhausted themselves in decades of mutual war and were riven by sectarian disputes their persecuted minorities resented. Conquered Christians and Jews became dhimmis, protected and taxed rather than forcibly converted, which made the new order tolerable and cheap to administer. Islam 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 under the Abbasids, Córdoba under the Umayyads of Spain, Cairo under the Fatimids, Samarkand on the Silk Road — preserved Greek philosophy, invented algebra, advanced medicine and optics, and held the line of the world's literacy when Latin Europe could not read its own past. That this happened under a faith barely two centuries old, in cities the conquerors had often founded themselves, is the measure of how thoroughly the new order absorbed and surpassed what it inherited.
Islam is now the religion of about two billion people across more than fifty states, the world's fastest-growing major faith and a majority on three continents. Many of the most consequential political fault-lines of the next decades — in the Middle East, in South Asia, in Europe's immigrant-shaped cities — sit on top of unresolved questions about how a 7th-century revelation accommodates 21st-century pluralism: the relation of sharia to secular law, of clerical to elected authority, of the umma to the nation-state. The argument is internal to the tradition, contested between reformers and traditionalists across Cairo, Tehran, and Jakarta, and it is not finished.