This book examines Euhemerism - a theory of religion developed by Euhemerus of Messene in the late 4th through early 3rd centuries BCE - tracing the history and reception of the theory in antiquity and in early Christianity, where it was adopted and reshaped for particular religious and ideological reasons. It shows how and why Euhemerism arose, how it was used, and how it significantly influenced theoretical discussions of religion from antiquity onward. Scholars have identified the euhemeristic tradition in various ancient and contemporary religious phenomena around the world: from the worship of the ancestors in the traditional African religions and the deified kings of the Mesoamerican civilizations to the Japanese Emperor in modern day Japan. This book examines those phenomena alongside the ancient theory of religion itself, including the motivations for its adoption, alterations of the theory in later periods, and conversion and identity formation. An Ancient Theory of Religion assumes no prior knowledge of Euhemerism and will be of interest to scholars working in classical reception, religious studies, and early Christian studies.