Within the field of American religious studies, a growing area of scholarship has been that of "spirituality" as a category distinct from religion. Scholars have examined the sociological, cultural, and historical features that characterize Americans' use of the concept of spirituality. Within this field, one subject of study is the growth in the number of individuals who identify themselves as "spiritual, but not religious." This phrase is used to denote a rejection of organized or traditional religion and an interest in a variety of belief systems. Via ethnographic methods, this dissertation analyzes this self-styled identity in the context of two phenomena: the Protestant legacy in the United States and "engaged spirituality," in which individuals' spirituality is integrally linked to engagement with social activism. The early Protestant history of the United States and the "Protestant ethic," per Max Weber, have shaped how Americans define and perceive religion and how Protestant values persist as cultural norms. American "spiritual, but not religious" individuals who are also "engaged" reject organized religion and find activism necessary due to issues that originate in this Protestant legacy. Evidence for this can be found in cases in which these individuals participate in activism by collaborating with non-Protestant religious groups. In this dissertation, I present this finding through three case studies featuring two radical religious groups which are active in peace protests: Nipponzan Myohoji, a Japanese Buddhist monastic order, and the Catholic Worker, a lay movement that assists the poor and homeless. The case studies are: the 50th anniversary Selma to Montgomery Civil Rights March; Catholic Worker protests in Washington, DC, on the anniversaries of the Hiroshima and Nagasaki nuclear bombings; and events at the Buddhist Great Smoky Mountains Peace Pagoda. I argue that these individuals form these alliances because in working with a Catholic and/or Buddhist group, they find a venue for activism which both accommodates their spiritual motivations and includes a critique of the Protestant-based elements of American culture.