"Keen insight…reveals existentialism as one more chapter in Christianity's history."— Journal of the American Academy of Religion The Religion of Existence reopens an old debate on an important question: What was existentialism? At the heart of existentialism, Noreen Khawaja argues, is a story about secular thought experimenting with the traditions of European Christianity. This book explores how a distinctly Protestant asceticism formed the basis for the chief existentialist ideal, personal authenticity, which is reflected in approaches ranging from Kierkegaard's religious theory of the self to Heidegger's phenomenology of everyday life to Sartre's global mission of atheistic humanism. Through these three philosophers, she argues, we observe how ascetic norms have shaped one of the twentieth century's most powerful ways of thinking about identity and difference—the idea that the true self is not simply given but something that each of us is responsible for producing. Engaging with many central figures in modern European thought, this book is of value to philosophers and historians of European philosophy, scholars of modern Christianity, and those working on problems at the intersection of religion and modernity.