Both Bonhoeffer and Barth find in history periods of time in which mankind made a fatal (kind of) move away from God – or at least the first steps of a journey away from God which led to the crisis(es) of faith that occupied both men in the 20th century.
For Bonhoeffer, it was the thirteenth century in which man began to become autonomous – through a series of theological, ethical, political, philosophical and scientific developments spanning the centuries. Various thinkers from Machiavelli, Descartes, Bodin, Spinoza, Herbert of Cherbury, Nicholas of Cusa, and Bruno all contributed to the removal of God as a working hypothesis in everyday life in their respective arenas.
For Barth, it was eighteenth century man that was the ‘absolute man’, – man at his most prideful, at the height of his non-need of God. For a more detailed look at Barth’s philosophy of history, I found this article by Derek Rishmawy to be very insightful:
For Bonhoeffer, it was these ideas that laid the groundwork for the coming-of-age of the world to where it finally was ready to leave God and religion behind – and as I’ve elaborated before, Bonhoeffers response was to attack the idea of making room for God, even in the form of ‘ultimate questions’ and instead recognize that God was and had to be at the very center of life.