This dissertation is a study of Paul Tillich's early theological development from his student days until the end of the First World War, set against the backdrop of church politics in Wilhelmine Germany and with particular reference to his early sermons. The central questions are: Where did Tillich 'land' theologically after the war? And how did he get there? The project offers a different focus from the majority of Tillich scholarship which understands Tillich primarily as a philosophical theologian. For before and during the First World War, Tillich was Pastor Tillich. Notwithstanding his philosophical study and academic ambitions, in the period 1904-18 he was studying theology to become a pastor, leading a Christian student group, working periodically as a pastor in Berlin churches, and preaching to soldiers. Through exposition of Tillich's early sermons, the pastoral context of his early thought becomes clear. Alongside the chronological boundary of 1919, the account is limited to one central theme, Tillich's notion of the justification of the doubter. In 1919 Tillich wrote an essay, Rechtfertigung und Zweifel [Justification and Doubt], for the faculty in Berlin, claiming the main problem of his systematic thought was the religious dialectic of doubt. This 1919 essay is compared with Tillich's writing across genres before and during the First World War. The influence of Schelling on Tillich's theology is well established. This dissertation also explores the role of Søren Kierkegaard, Ernst Troeltsch, Martin Kähler, Wilhelm Herrmann, Wilhelm Lütgert, Fritz Medicus and Tillich's father Johannes Tillich, distinguishing between Tillich's later autobiographical statements and the witness of archival sources. Drawing on material largely unknown to English-language scholarship, this chronologically and thematically focused dissertation on the issue of doubt in Tillich's early theology offers a significantly original, contextualised account of Tillich's early life in Germany. It should therefore be of interest not only to Tillich scholars, but to any interested in the history of modern theology, as an example of how biography and theology are intertwined.