Bonhoeffer's academic work is neglected in Bonhoeffer scholarship. In this study, J.I. de Keijzer aims to contribute to a better understanding of Bonhoeffer by examining the intellectual roots of his Act and Being, a notoriously inaccessible book, but one crucially important for grasping Bonhoeffer's theology. The author begins with an examination of Bonhoeffer's dialogue with Barth to find out how both theologians interacted with Luther's theologia crucis. The conclusion that Bonhoeffer deviates significantly from Barth's theological trajectory leads to another discussion in Act and Being, this time with Heidegger. J.I. de Keijzer shows how Bonhoeffer borrows from Heidegger's ontology to articulate an alternative theologia crucis that is characterized by a greater fidelity to Luther and a theological method that brings Christology, epistemology, hermeneutics, ecclesiology, and ethics together.