Tillich's main contribution to theology was to offer an account of the significance of human agency within the history of God's salvation of the world. Amidst the hopelessness of a world twice at war, Tillich introduced an apologetic theological approach, which he deemed a necessary counterpoint to the resulting despair of existentialism on one hand and the reaction of religious fundamentalism on the other. While some historical and contextual introduction is provided, this book is focused on analyzing, clarifying and connecting the most central and difficult of Tillich's theological concepts.