This dissertation is a study of the role of Protestant faith in six American boarding schools: Phillips Andover and Phillips Exeter (Congregationalist), Lawrenceville (Presbyterian), St. Paul's (New Hampshire) and Groton (Episcopalian) and Mount Hermon (nondenominational). Each of these schools began as a denominational institution with an explicit Protestant mission and purpose. However, competing missions were also present from their beginnings, particularly the imperatives of college preparation and of preparing good citizens for the Republic. As these competing missions grew in importance, the Protestant mission and purpose also changed. Intellectual and religious currents in the wider culture also forced a recasting of the role and scope of Protestant faith in their educational mission. In the early nineteenth century, Phillips Andover and Phillips Exeter each responded to the rise of Unitarianism in significantly different ways, Exeter by embracing it and Andover by reacting against it. Lawrenceville was powerfully affected by the Revival of 1857. The Gilded Age and Progressive Era proved particularly significant as the schools made an accommodation with modernity which downplayed the traditional Protestant piety in favor of a more generic Christian faith emphasizing moral behavior. Though elements of this accommodation first appeared at St. Paul's, it was fully implemented at Groton School after 1884, with the other schools adopting variations of it. (Mount Hermon proved to be an exception, maintaining a more evangelical faith.) Uneasy with the implications of a wholesale abandonment of traditional Protestantism, the schools adopted a blended faith, featuring a mix of traditional Protestant piety, nonsectarianism, and a generic morality of rectitude. This Gilded Age accommodation with modernity lasted until after the First World War, when the cultural crisis of the 1920s brought the arrangement into question. The headmasters of that era ended their careers on the eve of World War II harboring significant doubts as to the role that Protestant faith should, or could, play in the schools. Research for this dissertation was conducted in school archives accompanied by a wide reading in the secondary literature of the history of American Protestantism and the history of boarding schools.