American musical theater is often dismissed as frivolous or kitschy entertainment. But what if musicals actually mattered a great deal? What if perhaps the most innocuous musical genre in America actually defines the practices of Mormonism--America's fastest-growing religion? I address these questions in this dissertation by applying methodologies from Musicology, Voice Studies, Performance Studies, and American Studies to the unexpected yet dynamic relationship between Mormonism and American musical theater. In the introduction, I discuss how speaking on behalf of another person is rooted within both Mormonism and musical theater and argue for its critical examination. In Chapter 1, I argue that Mormonism and American musical theater are cut from the same ideological cloth and that musicals are intertwined with Mormon theology. Chapter 2 looks at the ways Mormons used musical theater as an assimilative tool during the mid-twentieth century. Chapter 3 looks at how Mormons used musical theater to "whiten" dark-skinned Polynesians and Native Americans, converting them, in essence, to Mormonism. In Chapter 4 I argue that the Mormon musical Saturday's Warrior helped Mormons bypass the strict system of order within Mormonism called Correlation. In Chapter 5, I suggest the Broadway musical Book of Mormon can be read as a critique of Correlated Mormonism. My investigation led to my conclusion that voice and vocal theatricality are deeply rooted concepts in Mormonism, to such a degree that I claim Mormons practice a theology of voice. I argue that analyzing the Mormon relationship with musicals from the mid-nineteenth century to today helps conceptualize the phenomenon of speaking on behalf of another person (or God), a process I call the vicarious voice. By closely examining voice in Mormonism and musical theater, I construct a theoretical approach to the performance of vocal vicarity. I conclude that, in an effort to become gods themselves, Mormons use the musical stage to practice transforming into someone they are not, modeling closely the theatrical qualities of Jesus and other spiritual leaders in Mormon mythology. Thus, learning to vicariously voice another person on the musical stage actually draws the faithful closer to godliness.