Why does the Dome of the Rock look like a Byzantine church?

The Dome of the Rock looks like a Byzantine church because, in a sense, it was built in the same visual language. The Umayyad caliph Abd al-Malik ordered it in 691–692 CE, over a sacred rock in Jerusalem, and it became the world's oldest surviving work of Islamic architecture. Its plan and mosaics were patterned after nearby Byzantine churches and palaces, and its builders reportedly used the measurements of the Church of the Holy Sepulchre. The caliph employed expert workers from across his domain, who were presumably Christians.

So this early Islamic monument borrowed the grand shapes of its surroundings: an octagonal plan, a dome on a drum, rings of arches around a central holy rock. What was new was the message. The building contains the earliest inscriptions proclaiming Islam and Muhammad, and inscriptions on later Islamic buildings follow the idea.

Why did the caliph build it? Medieval writers disagree. At the time he was fighting Christian Byzantium and a rival caliph who controlled Mecca. One explanation is that he wanted a monument that set Islam apart in a city of older faiths; a modern historian, Shelomo Dov Goitein, argued its round shape was meant to rival the Christian domes. Another says he wanted to pull pilgrims away from Mecca, but most modern historians dismiss that as anti-Umayyad propaganda.

The building also has a little detective story in its inscription. Long after it was finished, the caliph's name was removed and replaced with that of a later Abbasid caliph, al-Ma'mun. But the date, 72 AH, was left alone, and the swap was first noticed by a French scholar, Melchior de Vogüé, in 1864.