Conversion History: Middle Ages
In the face of persecution and legal restriction, codifiers of Jewish law chose survival over proselytization.
The political status of Jews in the Middle Ages, essentially subordinate to the Christian and Muslim authorities under whom they lived, combined with the continuing illegality of conversion to Judaism to prevent many conversions.
There were a wide variety of legal prohibitions that supplemented those imposed earlier. Between 395 and 408, the Byzantine Emperor Arcadius reenacted Constantius' decrees prohibiting proselytizing by the Jews; in 538 and 548, the Third and Fourth Councils of Orleans forbade Jews to proselytize; between 717 and 720, Omar II forbade Jews to seek converts among Muslims; and in 740, Egbert, the Archbishop of York, in England, forbade Christians to attend Jewish festivals. Under such circumstances, conversions continued on an individual basis, with mass conversions occurring only during those rare moments when the political status of the Jews was improved.
Conversion of Khazars Encourages Sense of Jewish Mission
Conversions occurred more frequently in the areas that were contiguous to Christian or Muslim rule, suggesting that conversion was, in part at least, a political strategy to resist the religious intrusion of Christianity or Islam. The two most famous cases of this are the conversion of Yusuf Dhu Nuwas, the King of Himyar (in what is currently Yemen) in the early part of the sixth century and the conversion of the Khazar royal house in the 720s.
Dhu Nuwas, attempting to achieve freedom from Christian Abyssinian rule, could not get the assistance he sought from Persia, and his Jewish kingdom did not survive. The Khazars were a Turkic people who lived between the Black and Caspian Seas in Southern Russia. Legend has it that King Bulan held a debate among speakers for Judaism, Islam, and Christianity and chose on the basis of what he heard to accept Judaism. It is more probable, however, that Jewish traders, travelers, and refugees introduced Judaism to the kingdom. Khazaria eventually fell, and some of its Jews went to Eastern Europe.
What is most important about the Khazars was their effect on medieval Jews. The rise of Christianity and Islam, following on the loss of sovereignty and the concomitant demise in any military or political power, had made Jews desperate for a theological explanation of their plight. The explanation that they would one day be justified when the messiah came and that in the meantime they would be judged by their adherence to the mitzvot [commandments] and not by temporal success had not yet fully seeped into the Jewish imagination.