Isaac Orobio de Castro, a crypto-Jew from Portugal, was one of the most prominent intellectual figures of the Sephardi Diaspora in the seventeenth century. After studying medicine and theology in Spain, and having pursued a distinguished medical career, he was arrested by the Spanish Inquisition for practising Judaism, tortured, tired, and imprisoned. He subsequently emigrated to France and became a professor of medicine at the University of Toulouse before openly professing his Judaism and going to Amsterdam where he joined the thriving Portuguese Jewish community. Amsterdam was then a city of great cultural creativity and religious pluralism where Orobio found open to him the world of religious thinkers and learned scholars. In this atmosphere he flourished and became an outstanding spokesman and apologist for the Jewish community. He engaged in controversy with Juan de Prado and Baruch Spinoza, who were both excommunicated by the Portuguese Jewish community, as well as with Christian theologians of various sects and denominations, including Philip van Limborch.This fascinating biography of Orobio sheds light on the complex life of a unique Jewish community of former Christians who had openly returned to Judaism. It focuses on the particular dilemmas of the converts, their attempts to establish boundaries between their Christian past and their new identity, their internal conflicts, and their ability to create new forms of Jewish life and expression.