Answer
The 1930s marked the significant evolution of Reinhold Niebuhr's theological perspective. During this period, he transitioned from his earlier liberal Protestant views to a more robust and realistic theological framework. Niebuhr's newfound emphasis on human sin and the limitations of human reason led him to formulate his influential theory of Christian realism. This theological shift is evident in his magnum opus, "The Nature and Destiny of Man," published in 1941.