Theologian in BaselIn 1870, Overbeck became professor of New Testament Exegesis and Old Church History at the University of Basel. From that time until 1875, he lived in the same house (one floor under) as his colleague Friedrich Nietzsche, the Professor of Classical Philology at the same university. During this time, the housemates developed a friendship that would remain crucial for each other.
In 1873, Overbeck published his most important work 'How Christian is Our Present-Day Theology?' (Über die Christlichkeit unserer heutigen Theologie), in which he argued that the "historical" Christianity, as developed by the fathers of the church, neither did nor could have to do with the original ideas of Christ. He observed that early Christianity had opposed itself to every type of history, culture, and science, which made a "Christian theology" impossible. In this work, Overbeck criticized the conservative ("apologetic") theology, which stuck dogmatically to doctrines, as much as the "liberal" theology, which asserted that belief and knowledge could be reconciled. According to Overbeck, both failed to capture an essence of Christianity, which excludes every type of scientific knowledge.
This work was primarily incited by David Strauss's The Old and New Faith (Vom alten und neuen Glauben, 1872) and Paul de Lagarde's 'On the Relationship of the German State to Theology, Church, and Religion' ('Über das Verhältnis des deutschen Staates zu Theologie, Kirche und Religion', 1873). Both authors attempted to shape a modern Christian religion with the help of theological scholarship. Overbeck regarded this project as impossible and fundamentally in error. In his afterword for the second edition in 1903, he renewed this critique against theologian Adolf von Harnack and his work 'The Essence of Christianity' ('Das Wesen des Christentums', 1900).
The publication of this book practically destroyed all his chances to become professor at a German university. He remained in Basel, and for more than ten years, he held the same introductory lecture without addressing his provocative theses.
In private, Overbeck made voluminous notes for a 'Church Lexicon' (Kirchenlexikon), in which he develops personal accounts, principally theological but also political, cultural, philosophical, and a literature bibliography with commentary. The goal of this collection fulfilled the only purpose Overbeck saw for a scholarly theologian: a profane history of the church. Exactly what Christianity itself would not explain or could not understand, moreover what it would deny, Overbeck documented, thereby demonstrating his primary dilemma: that a "Christian theology" is impossible.
In 1876, Overbeck married Ida Rothpletz, and for one year was Rector of the University of Basel.