Posthumously issued, this volume consists of material taken directly from the Robertson Lectures, delivered at the University of Glasgow in 1960 by the late professor of theology at the Yale Divinity School. Their issue is briefly accounted for in a preface by Professor: Niebuhr's son, and their theme is placed in its setting in the development of his thought by his junior colleague, James Gustafson. Additional material on the theme is appended from the Earl Lectures. Niebuhr had looked forward to his retirement from teaching as the time for drawing together in definitive form the long-matured reflection and thought which had taken shape through his years of distinguished teaching and scholarship. This hope was denied by his death; there is, therefore, the more reason for gratitude for the issue of this volume which, in admittedly incomplete form, brings some of the strands of that thought together.