Until the 18th century, there was basically only one kind of Judaism, that which is now called Orthodox. It meant living by the religion's 613 laws, and doing so suffused Jews' lives with their faith. Then, starting with the thinker Baruch Spinoza (1632-77) and moving briskly during the Haskala, or "enlightenment," from the late 18th century, Jews developed a wide variety of alternate interpretations of their religion, most of which diminished the role of faith in their lives and led to a concomitant reduction in Jewish affiliation.These alternatives and other developments, in particular the Holocaust, caused the ranks of the...