This study of the Hebrew treatise Kelimat ha-Goyim (“Shame of the Gentiles,” 1397) by Profiat Duran exemplifies the stimulating impact medieval religious polemics exerted on the scholarly construction of Christian religious history. Besides explaining Jesus in his Jewish context, this Catalan author outlined in detail the emergence of the fundamental Christian dogmas during the apostolic, patristic, and medieval age and searched for the driving forces behind long-term religious transformation. While a common view holds that Duran’s method of New Testament study mirrored thirteenth-century Christian Talmudism, I underscore his originality as a historian of religion, whose clandestinely transmitted text still inspired early modern and nineteenth century attempts at critical scholarship. Duran’s proper context is the contemporary converso problem. A comparison with a Spanish Renaissance text, the Diálogos en Marruecos, strongly suggests that his historical representation of ex-Jews turned into Christian leaders not only addressed conversos, but actually meant to caricature them.