Introducing Historical Approaches to Leaving Religion In the early fourteenth century, the Castilian Jew Abner of Burgos (d. ca. 1347) embraced the Christian religion and began to call himself Alfonso of Valladolid after changing his city of residence. He passed the remaining three decades of his life writing extensive polemics against Judaism, not in Latin, but in Hebrew, some of which he and possibly others translated into Castilian. Although there is no evidence that the sincerity of his conversion was ever in question among his Christian brethren, Abner/Alfonso-he is known by both names, depending on the sources-became one of the Christian anti-Jewish polemical writers who was most often cited by Jews, a majority of whom roundly condemn him as a liar, traitor, and apostate (Szpiech 2013: 144-150). Abner/Alfonso's reputation among Jews as a notorious apostate has persisted to the twentieth century. Historian Yitzhaq Baer, states, "Abner did not belong in the fold of orthodox Catholicism; he was a typical mumar, a heretic within the Jewish camp" (Baer 1961: 1:334). Yet even though his "heresy" was commonplace, his reputation was not, for as word of his actions spread, he became no less than "the best known apostate ever to arise in medieval Jewry" (328). For Baer, Abner/Alfonso's identity as an apostate relies on the lingering persistence of his Jewish identity, his remaining within the Jewish fold rather than leaving it. For Baer, Abner/Alfonso "remained a Jew at heart." (334). Yet not all historians agree, and some see him as a traitor who left Judaism (for example, Shamir 1975: 53, who deems him a "true convert"). Raphale Jospe sums up the problem, posing the question, "Do we treat such thinkers and their works as Jewish, as Jewish until the time of their apostasy, or as non-Jewish and having no place in the context of Jewish philosophy?" (Jospe 1988: 9, my translation). If Abner/Alfonso was a "Jew at heart" who remained "within the Jewish camp," was his departure from Judaism insincere and incomplete? Must he be deemed a "true convert" (and so also a "true apostate") in order to "leave" one religion for another? Abner/Alfonso's life and works present historians with a vexing question about scholarly methodology: how can one approach without bias the subject of leaving religion in historical sources? The question springs from a broader © Ryan Szpiech, 2020 | doi 10.1163/9789004331471_022 This is an open access chapter distributed under the terms of the CC BY-NC 4.0 license.