The rise of Islam presented a major challenge to Orthodoxy. Beginning in the seventh century, major portions of the Orthodox heartlands (in Syria and Egypt) fell under Muslim rule. By the fifteenth century, most traditionally Orthodox lands were controlled either by Muslim or Roman Catholic rulers, with the exception of northeastern northwestern Russia (the Grand Principality of Moscow) and the Ethiopian highlands. The Orthodox generally accepted rule by non-Orthodox governments, provided that freedom of worship was guaranteed. In Ottoman lands, governed under the ''[[w:Millet (Ottoman Empire)|millet]]'' system (by which people were grouped by religion rather than nationality), Orthodox bishops also served as ''Ethnarchs'' (political rulers of their communities).<ref>Dr. Catharine Cookson, (J.D., Ph.D., 1952-2004). [http://books.google.ca/books?hl=en&id=R0PrjC1Ar7gC&dq=encyclopedia+or+religious+freedom&printsec=frontcover&source=web&ots=8wJLZH8mcI&sig=CpzOJuISIQXQRgEowBQe-iz1QO8&sa=X&oi=book_result&resnum=6&ct=result Encyclopedia of Religious Freedom]. Published by Taylor & Francis, 2003. pp.313.</ref> Ottoman implementation of the [[w:Devşirme|devsirme tax]] system witnessed Orthodox children of the rural populations of the Balkans, the flower of Orthodox Christendom, conscripted before adolescence and brought up as Muslims. As their empire declined, the Ottoman Muslims became decreasingly tolerant of Orthodox Chrstians.