This dissertation argues that in the heyday of radio, religious-radio preachers sought to use their programs to counter the secularization of American culture. Ultimately, however, their programs contributed to secularization by accelerating changes already evident in both the conservative and liberal streams of American Christianity. To reach a vast American audience, radio preachers transformed their sectarian messages into a religion more suitable to the masses, thereby altering the very religion it aimed to preserve. This was one of the unintended consequences of American religious radio. Chapter 2 argues that Harry Emerson Fosdick's ministry contributed to a movement away from Protestant orthodoxy. Radio played an important role in Fosdick's successful effort to blaze a new theological trail for the modern era. Chapter 3 shows that Aimee Semple McPherson's experiential religion had an ecumenical appeal that reached areas across the world. As one of the most celebrated Christian figures of the early twentieth century, McPherson's ministry helped make the American church more accepting of important aspects of secularization. Chapter 4 argues that Walter Maier's ministry encouraged millions of people to believe in a simple orthodoxy. The message he broadcast eschewed Lutheran particulars for the bold proclamation of the basic convictions shared by Christian fundamentalists generally. Chapter 5 argues that Charles Fuller's ministry contributed to the transformation of American religion by defining it primarily in terms of evangelism. His success uncovered a particular mood in America: one tired of the militant fundamentalism of the early decades of the century but not ready to abandon the fundamentals of the faith for theological liberalism. Chapter 6 considers religious radio against the backdrop of the emergence of television as the dominant communication medium in America and draws out implications for religion in the modern age of the Internet. To make religion accessible to large and diverse audiences, radio preachers accommodated their messages in ways suited to the medium of radio. Although religious-radio preachers set forth to advance the influence of religion in American society, their choice to limit theological substance ironically promoted the secularization of the American church.