This dissertation explores how Philadelphia Catholics of the early national period sought to reconcile the conflicting forces of spiritual expression, American citizenship, and Protestant antipathy in their quest to establish an American Catholic identity. Previous historians have posited that, by the middle of the nineteenth century, a colonial and early national Catholic identity, articulated by mostly native-born American laypeople and rooted in Enlightenment and republican values, yielded to a European, Ultramontane vision of Catholic community life. It has been assumed that clergy succeeded in squelching lay-led campaigns for ecclesiastical democracy and achieved widespread acquiescence to a more elaborate, authoritarian Church hierarchy as well as a more separatist orientation to the broader Protestant American culture. This study revises the prevailing historiographical formulation by revealing the contributions of the American laity to the reorganization of devotional and parish life in the middle decades of the nineteenth century. It also explores Catholics' continued engagement with their Protestant neighbors. The experiences of Philadelphia Catholics demonstrate that collaboration between clergy and laity was crucial to the articulation and implementation of a common spiritual and social identity. Special attention to three key developments in the history of early national Catholicism shed light on how conflicts and collaborations between laypeople and their clerical superiors propelled American Catholic community formation. First, fruitful partnerships between clergy and Catholic publishers made it possible for the faith community to deploy a vibrant culture of print to bolster piety and to connect co-religionists both within and beyond Philadelphia. Second, disagreements over the future of trusteeism (the traditional means of founding and managing new parishes) forced Catholics to renegotiate the balance of power between clergy and laity. Finally, Catholics found it necessary to launch compelling apologetic campaigns in the face of rising nativist hostility. Clergy and laity cooperated to overturn popular misconceptions of Catholic doctrine and to demonstrate the community's respectability in the eyes of Protestants. When taken together, these sites of inquiry reveal that both the clergy and the laity were vital contributors to the creation of a distinctive American Catholic identity.