This contribution to the Common Knowledge symposium “Fuzzy Studies” argues, on the basis of recent research, that religious polemic is a phenomenon closely associated only with monotheist traditions. Focusing on religious polemics in medieval and early modern Islamic and Christian Spain, it analyzes polemical texts of diverse natures and from different centuries to see how their authors, by attacking both dogmatic and legal opinion, aimed to harden the amorphous boundaries between groups. On the Christian side, polemicists argued for the restriction of the rights of Muslims and Jews in Christian territories, or for the outright expulsion or conversion of these groups. On the Islamic side, a number of Muslim authors voiced the claim that Muslims should emigrate from Christian territories. The article seeks to show how these polemical voices argued against known opponents but also against anonymous Christians, Muslims, and Jews who, untouched by such polemics, had lived close to one another and, without conflict, had accepted the undefined character of their daily life together. In support of this argument, the author makes use of hitherto unknown archival documents regarding how adherents of the three religions interacted in medieval Christian Spain.