Mark Twain is often pictured as a severe critic of religious piety, shaking his fist at God and mocking the devout. Such a view, however, is only partly correct. It ignores the social realities of Twain's major period as a writer and his own spiritual interests: his participation in church activities, his socially progressive agenda, his reliance on religious themes in his major works, and his friendships with clergymen, especially his pastor and best friend, Joe Twichell. It also betrays a conception of religion that is more contemporary than that of the period in which he lived. Harold K. Bush Jr. highlights Twain's attractions to and engagements with the wide variety of religious phenomena of America in his lifetime, and how these matters affected his writings. Though Twain lived in an era of tremendous religious vigor, it was also a time of spiritual upheaval and crisis. The rise of biological and psychological sciences, the criticism of biblical texts as literary documents, the influx of world religions and immigrant communities, and the trauma of the Civil War all had dramatic effects on America's religious life. At the same time mass urban revivalism, the ecumenical movement, Social Christianity, and occultic phenomena, like spiritualism and mind sciences, all rushed in to fill the void. The rapid growth of agnosticism in the 1870s and 1880s is also clearly reflected in Twain's life and writings. Thus, Twain's career reflects in an unusually resonant way the vast changes in American belief during his lifetime. Bush's study offers both a new and more complicated understanding of Twain and his literary output and serves as the cultural biography of an era.