In this thesis I argue that unconscious Christianity (unbewußtes Christentum), referred to by Dietrich Bonhoeffer in several of his later writings, is a significant idea in his late theology. There has as yet been no in-depth study of this theological concept as it appears in Bonhoeffer's work, and I therefore aim with this thesis to begin a new conversation in Bonhoeffer studies on this important topic. Bonhoeffer does not offer a definition of unconscious Christianity, but by analysing the ways in which he uses the term in his writing, I offer a constructed definition of unconscious Christianity as used by Bonhoeffer. The first three chapters of the thesis build towards this definition with a close analysis of each relevant text. By examining unconscious Christianity alongside other theological ideas in Bonhoeffer's prison writing, I show how an awareness and understanding of unconscious Christianity adds depth to readings of Bonhoeffer's late work. This thesis also clarifies the differences between unconscious Christianity and religionless Christianity, and shows how unconscious Christianity fits alongside the other, more widely-studied, concepts present in the later writings, such as the world come of age. This work demonstrates that there is movement within Bonhoeffer's thoughts on unconscious Christianity and points to Bonhoeffer's readiness to allow his personal circumstances to inform his theology. It also shows how unconscious Christianity represents a shift within Bonhoeffer's theology. This thesis also makes the subsidiary point that Bonhoeffer's prison fiction should be considered as theological writing. Through it Bonhoeffer addresses not only unconscious Christianity as discussed in this thesis, but many other issues that reoccur in his theological prison letters. I conclude by showing how an understanding of unconscious Christianity is beneficial not only for Bonhoeffer studies, but for contemporary theology more widely.