For more than five centuries after Justinian, the Roman Empire in the East was engaged in a long struggle with the foes—Persians, Arabs, and Seljuk Turks—which successively attacked its dominions. By its stubborn resistance of the advance of the invaders the old empire protected the young states of Europe from attack, until they grew strong enough to meet and repulse the hordes of Asia. This service to civilization was not less important than that which had been performed by Greece and Rome in their contests with the Persians and the Carthaginians.
Christianity reached the invaders of the Balkan peninsula from Constantinople. The Serbians, Bulgarians, and Russians were converted in the ninth and tenth centuries. With Christianity they received the use of letters and some knowledge of Roman law and methods of government. Constantinople was to them, henceforth, such a center of religion and culture as Rome was to the Germans. By becoming the teacher of the vast Slavic peoples of the Balkan peninsula and European Russia, the empire performed another important service to civilization.
The Roman Empire in the East, though often menaced by barbarian foes, long continued to be the leading European power. Its highest degree of prosperity was reached between the middle of the ninth and the middle of the eleventh century.
From The Making of Europe / Early European History