When did the first genocide take place in history? In theory, a universal crime transcends time and space. In practice, the moral imagination demands a specific origin story. In this article, I explain how and why Raphael Lemkin chose to locate genocide’s archetypal origins in the early Christian martyrdom at the hands of the ancient Romans. That choice emerged from a dramatic public confrontation with Catholic antisemitism in interwar Poland. Haunted by the charge of Jewish moral parochialism, after the war Lemkin fashioned a cosmopolitan narrative for his discovery of genocide. Today, scholars are consumed by debates about the historical and conceptual relationship between the Holocaust and other genocides. Yet we cannot move forward in that endeavor until we retrieve Lemkin’s Polish Jewish past.