This thesis examines the religious influences on Louis Henri Sullivan (1856-1924), a key figure in the birth of American modernist architecture and a prolific writer on architecture’s relationship to nature, democracy, and individual genius. Its argument is that, although implicated in and to a great extent beholden to industry’s effects on Chicago’s built environment, Sullivan was intent to raise the stakes of architectural practice and of urban aesthetics more broadly beyond crude materialism, and to do so he used transcendentalist discourses already at work in the American architectural world. With the concept of form as its key way of explaining how Sullivan connected transcendentalists’ literary productions to his material practice, this thesis charts the genealogy of Sullivan’s theory of architecture and shows how Sullivan made use of those theories in his architecture and in writings crucial to his lasting reputation.