The four canons of the Fourth Lateran Council of 1215 regarding Jewish matters drew heavily on earlier ecclesiastical material and touched on fundamental issues concerning Christian-Jewish relations. Subsequently they were included in Gregory IX’s definitive collection of canonical material, the so-called Decretals, of 1234. As such they constitute an excellent platform from which to embark on an in-depth examination of medieval Christian policies and doctrines concerning Jews and Judaism. The topics covered by the canons included the complexities surrounding Jewish conversion to Christianity, the vexed problem of Jews having any kind of authority over Christians, Christian concerns about Jews and Judaism contaminating Christian society and mocking the Christian faith and as well as the fraught issue of Jewish usury (in the medieval sense of charging interest) and Jewish liability for tithes. The analysis of ecclesiastical rulings on these issues demonstrates how ambiguous ecclesiastical policies and doctrines on Jews were. Jews were excruciated for their lack of Christian belief; at the same time they were protected because they were deemed to play a theological role in Christian society by being Jewish. For lay rulers Jewish usefulness in providing linguistic, medical and administrative services and taxation on their economic activities often weighed more heavily than ambiguous theological considerations.