Anthonius Margaritha, the son of a rabbi, was a German Jew who converted to Christianity in 1522. He is best remembered for his Der gantz Jüdisch Glaub (1530), in which he described many Jewish customs, gave his German translation of the Hebrew prayer book, and presented a “refutation” of Judaism. The accuracy of his ethnographic data is crucial to using Der gantz Jüdisch glaub in scholarly discussion. This essay tests Margaritha’s assertions about German-Jewish physicians and about the anti- Christian polemic in the aleinu prayer against external Jewish and Christian sources. Margaritha is found to be a reliable reporter, albeit a biased one. The essay also adds to an understanding of early modern German-Jewish physicians and of the Jewish polemic against Christianity.