In the past half-century, the American social sciences have been eager to research Reinhold Niebuhr’s ideas. Most current works on Niebuhr deal with either what Niebuhr thought or its contemporary consequences. This has distracted the attention from the “sources” of Niebuhr, namely, what kind of theoretical influences affected him while shaping his political thought. This objection is valid even in the case of (Saint) Augustine (354-430), who – based on the relevant literature, Niebuhr’s intellectual autobiography, and his wife’s, Ursula’s Niebuhr statements – has had a significant influence on his thought. In a private letter, Niebuhr called himself a “critical student of Augustine.” This dissertation aims to fill a theoretical gap through a systematic elaboration and evaluation of Niebuhr’s interpretation of Augustine. The dissertation – by overviewing Niebuhr’s scholarly work – presents how Niebuhr interpreted Augustine and his ideas. The focus is on the content and context of Niebuhr’s thoughts on Augustine and his ideas, the different characteristics of Niebuhr’s theoretical phases, the reasons for changes, and the overall picture. The main body of the research is based on primary sources from Niebuhr; nineteen books were processed, most of which are monographs or compilations of essays and sermons. The initial novelty of the dissertation lies in the systematic effort to understand Niebuhr’s interpretation of Augustine throughout his life and scholarship. It also shortly refers to the gravest misunderstandings of Niebuhr’s interpretation of Augustine. Furthermore, it seriously considers the numerous influences (theoretical and historical) that changed Niebuhr’s perspective on Augustine (for instance, Charles Norris Cochrane’s impact). Ultimately, the incessant focus on the chronological aspect helped formulate grounded conclusions connected to Augustine, for instance, in the case of Christian realism.