In the long history of conversion to Judaism, few figures rival the affection and esteem with which Count Walenty (Valentine) Potocki, the martyred "Vilna Ger Tzedek," is remembered. The story of the young Polish nobleman, burned at the stake in 1749, is that of a principled and sensitive spiritual seeker who abandoned wealth, power, and all but unlimited worldly prospects in order to adopt Judaism, a religious tradition that was anathema to his noble class. Potocki was betrayed by a member of the religious community he embraced, and executed as an apostate by the Church he left behind, and...