The inquiry into the relationship between religion and modern rationality stands as one of the most significant contemporary philosophical issues and one of the most influential in shaping the comprehensive existential-cosmological view of contemporary human beings. This relationship encompasses cognitive and value-based implications that affect one's understanding of existence, the meaning of life, and the boundaries of reason and its role in organizing both individual and social experience. Habermas's intellectual project, despite its significance and breadth, reveals a clear methodological tension between its premises and its conclusions regarding religion. His prior commitment to secularism led him to adopt a reductionist view that confines religion to partial social and ethical functions, thereby disregarding and marginalizing its fundamental existential and cognitive dimensions. Although Habermas's later position evolved toward acknowledging religion's role in the public sphere, his fundamental theoretical framework within which this idea is tested remains secular and functional at its core, which prevents understanding religion as a comprehensive cosmological view. In the context of his analysis of modernity's trajectory, Habermas contends that religion's position has diminished in the modern era, which forms the basis for his conception of the relationship between religion and rationality. This article seeks to expose the deficiencies in this conception by analyzing and critiquing Habermas's arguments. Through an analytical and critical methodology, the study demonstrates that rationality, in all its forms and across various domains, remains incapable of replacing religion.