The sixteenth century witnessed the transition of medieval religious anti-Judaism into a racial antisemitism laying the foundation for modern hatred of Jews. This change resulted from the institution of the "pure blood laws" by the Spanish courts of Inquisition to determine who was and who was not Jewish. Such racial definitions were necessary because hundreds of thousands of Jews voluntarily converted to Christianity, indeed, even attempted to make religious contributions to their new religion, as the efforts of St. Theresa, Alphonso and Juan de Valdes, Luis Vives, Luis de Leon, and the Jesuits Salmeron and Laynez and Juan Alonso de Polanco, among many many others, must demonstrate. Indeed, even Protestant learning of Hebrew was predicated upon the efforts of Jewish converts such as Matthew Adrian, Cornelius Adelkind, and others. It was precisely this wholesale swallowing of former Jews and much Jewish linguistic and exegetical expertise that brought about Christian revulsion against "Jewish contamination" and the use of a biological standard for determining religious identification. This process was complicated by the active complicity of New Christian authorities, wishing to distance themselves from their former co-religionists, in the development of racial categories. While generations of historians have sought to explain Luther's antisemitism resulting from a frustration with Jewish unwillingness to convert to Christianity, all the evidence points in the other direction. Luther and others developed a more defined sense of antisemitism precisely because they believed they saw "Jewish"-i.e. New Christian, -influence, everywhere about them.
JSTOR: The Sixteenth Century Journal, Vol. 18, No. 1 (Spring, 1987), pp. 3-30