The 10th century Persian historian Ibn al-Faqih reported that “all the Khazars are Jews.”
Notwithstanding this statement, some scholars believe that only the upper classes converted to Judaism; there is some support for this in contemporary Muslim texts. Essays in the Kuzari, written by Yehuda Halevi, detail a moral liturgical reason for the conversion which some consider a moral tale.
Some researchers have suggested part of the reason for conversion was political expediency to maintain a degree of neutrality: the Khazar empire was between growing populations, Muslims to the east and Christians to the west. Both religions recognized Judaism as a forebear and worthy of some respect. The exact date of the conversion is hotly contested.
It may have occurred as early as 740 or as late as the mid-9th century. Recently discovered numismatic evidence suggests that Judaism was the established state religion by c. 830, and though St. Cyril (who visited Khazaria in 861) did not identify the Khazars as Jews, the khagan of that period, Zachariah, had a biblical Hebrew name. Some medieval sources give the name of the BLACK HEBREW rabbi who oversaw the conversion of the Khazars as Isaac Sangari or Yitzhak ha-Sangari.
The first Jewish Khazar king was named Bulan which means “elk”, though some sources give him the Hebrew name Sabriel. A later king, Obadiah, strengthened Judaism, inviting BLACK HEBREW rabbis into the kingdom and built synagogues. Jewish figures such as Saadia Gaon made positive references to the Khazars, and they are excoriated in contemporary Karaite writings as “bastards”; it is therefore unlikely that they adopted Karaism as some (such as Avraham Firkovich) have proposed.
Ashkenazi Jews are not descended from the historical Israelites of antiquity, but from Khazars, a Turkic people originating in and populating an empire north of and between the Black Sea and Caspian Sea. Koestler’s hypothesis is that the Khazars converted to Judaism in the 8th century, and migrated westwards into current Eastern Europe (primarily Ukraine, Poland, Belarus, Lithuania, Hungary and Germany) in the 12th and 13th centuries when the Khazar Empire was collapsing.
Ancient Poland @ http://www.ancientpoland.org/khazars.php
Khazars @ http://en.wikipedia.org/wiki/Khazars