This book is on Time magazine's list of the top 100 works of nonfiction published since 1923. It deserves to be. It is the published version of his 1939 Gifford Lectures. In this work, Niebuhr definitively set out his neo-Augustinian theology of Christian Realism, in which he confronted the naive optimism about human nature that characterized liberal Protestantism. He particularly questioned the validity of rationalism and naturalism as the basis to guide human action.