Petrus Alfonsi's Dialogue Against the Jews (ca. 1109) breaks new ground in the history of Christian anti-Jewish polemics. As a recent convert from Judaism, Alfonsi introduced an intimate knowledge of Jewish literature and contemporary practice absent from earlier Christian sources. This knowledge enabled him to attack for the first time the Talmud (or, more broadly, post-biblical Jewish literature) as a source of Jewish error, with arguments drawn from philosophy and theology, astronomy, medicine, and physics. Equally important, Alfonsi's Dialogue contains an extensive anti-Muslim polemic to explain not only why he abandoned Judaism but also why he rejected Islam and chose the Christian faith. For these reasons, the Dialogue has been described as the most important anti-Jewish text of the Latin Middle Ages. This assessment is based not only on its innovative argumentation but also on the fact that it was one of the most popular medieval anti-Jewish polemicswritten.
