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. / PHD / Beginning in the 1970s, the topic of spirituality as a distinct area of study has developed within the field of American religious studies. One subject within the study of spirituality is the growing number of individuals who identify themselves as “spiritual, but not religious.” This dissertation is an effort to further explore what roles these individuals play in American society. To accomplish this, this study addresses the relationship of “spiritual, but not religious” individuals to religion in the context of social activism. Through ethnographic methods of interviewing and participant-observation, this dissertation presents cases of spiritual but not religious individuals who identify activism as a key part of their spirituality. Specifically, these individuals participate in activism by collaborating with 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. Although “spiritual but not religious” individuals are defined by their rejection of organized religion, they choose to ally with these religious groups for the purpose of activism. I argue that these individuals form these alliances because the underlying social problems and elements of organized religion that they reject originate in the Protestant legacy in American culture. Therefore, in working with a Catholic and/or Buddhist group, these individuals find a venue for their spiritually-based activism that includes a critique of mainstream American culture.