The first decades of the twentieth century were one of the greatest periods of church building in American history. Although many ambitious large-scale ecclesiastical commissions were under construction, many tens of thousands more small- and medium-sized churches were also designed and erected. The dissertation examines this unique period of ecclesiastical building, amidst a broader context of American religious and secular architecture. It concentrates on the response by mainline Protestant denominations to a pressing need for new, efficient churches and the development of standards and controls for their design. It looks at the establishment of professional departments of architecture within individual denominations, the scientific means of investigation and survey that aided in the reassessment of the church building and the role of the church within contemporary society, and attempts at interdenominational cooperation on these matters. The dissertation argues that a new, modern church building type was established, which was part of an aggressive strategy for safeguarding and strengthening Protestant hegemony in a changing social and religious landscape. Furthermore, it argues that church building became an important forum for discourse on the state of the architecture profession and on the role of architectural standardization.