From The Art and Popular Culture Encyclopedia
"The cause of the break with the tradition of antiquity was the rapid and unexpected advance of Islam. The result of this advance was the final separation of East from West, and the end of the Mediterranean unity. Countries like Africa and Spain, which had always been parts of the Western community, gravitated henceforth in the orbit of Baghdad. In these countries another religion made its appearance, and an entirely different culture. The Western Mediterranean, having become a Musulman lake, was no longer the thoroughfare of commerce and of thought which it had always been." --Mohammed and Charlemagne (1922) by Henri Pirenne
The East–West Schism, or the Great Schism, divided medieval Christendom into Eastern (Greek) and Western (Latin) branches, which later became known as the Eastern Orthodox Church and the Roman Catholic Church, respectively. Relations between East and West had long been embittered by political and ecclesiastical differences and theological disputes.