Though not written from an explicitly Christian perspective, Moral Man and Immoral Society (MM), is one of the most, if not the most, influential works of Christian ethics in the 20th c. Originally published in 1932, from it springs the ethical, philosophical, sociological bases for Niebuhr's "Christian realism" fleshed out further in his more explicitly theological works. The most telling illustration of its significance, especially in Protestant circles, is that Christian ethicists from one level of appreciation of Moral Man to the other—from Niebuhr's fellow Christian realists to John Milbank in his essay "The Poverty of Niebuhrianism," along with many others somewhere in the middle (e.g., Martin Luther King, John Howard Yoder, Stanley Hauerwas), have had to engage Niebuhr and Moral Man in carving out their own positions. Catholics as well have sought to integrate Niebuhr's insights into their work, although Michael Novak's use of the egoism highlighted in Moral Man in his apologia for capitalism undoubtably would have Nieibuhr "turning over" in his grave.
It is fitting then that The Library of Theological Ethics has republished Moral Man and Immoral Society, along with an excellent introduction by Langdon Gilkey. Besides briefly analyzing Niebuhr's criticism of religion and reason as inherently reliable sources of social analysis and change, Gilkey helpfully locates Moral Man not only in its historical context but also in relationship with his later theological works, especially The Nature and Destiny of Man.
The focus of Moral Man is, as Gilkey puts it, though "individuals act with seeming morality, this does not mean that as members of their social groups—in class, racial, economic, or political matters—they in any way escape doing and supporting injustice." This leads to immoral group behavior trumping individual and social morality. In Niebuhr's words, "The inferiority of the morality of groups to that of individuals is due in part to the difficulty of establishing a rational social force which is powerful enough to cope with the natural impulses by which society achieves its cohesion....," that natural impulse being self interest. In other words, morality may be achieved in individuals and intimate social relationships, but in society the use of reason alone is unable to check the powerful interests of groups such as nations and even religions. Referring to his "rationalist" contemporaries such as John Dewey, Niebuhr argued, "Complete rational objectivity in a social situation is impossible. . . Since reason is always, to some degree, the servant of interest in a social situation, social injustice cannot be resolved by moral and rational suasion alone, as the educator and social scientist usually believes. Conflict is inevitable, and in this conflict power must be challenged by power." Niebuhr therefore strongly stresses the limits of rational, moral and even religious resources in promoting a just society. Not that social justice should not be attempted, and Niebuhr was definitely a strong voice and activist toward that end, but that we should be under no illusion that any victories over iniquities will be final or even substantial.
One of the strongest criticisms of Niebuhr in Moral Man has been of this pessimism regarding human nature and the possibilities for lasting social justice. As one of his students famously remarked, the real title of Moral Man and Immoral Society should have been "Somewhat Immoral Man and His Even More Immoral Society." This is exemplified in Niebuhr's view that, "civilization has become a device for delegating the vices of individuals to larger and larger communities." As someone more hopeful (realistic?) about reason and religion as resources toward individual and social transformation, I also have found points to disagree with in Moral Man. Despite these and other criticisms, Moral Man and Immoral Society and Niebuhr's Christian realism have provided an effective, lasting, and needed counterpoint to any easy optimism or unreflective and uncritical attempts toward social change, even and especially in the name of religion or "freedom." Also, while realizing that we live in a more pluralistic time than Niebuhr's, it seems the more Christian ethics moves away from Niebuhr's positions to more sectarian ones, the farther the Christian community and its thinkers unfortunately get away from the active critical engagement between Christian ethics and "immoral society" that Niebuhr exemplified in his writings and in his actions.