Christian theologians Karl Barth (1886-1968) and Dietrich Bonhoeffer (1906-1945) inherited a particular understanding of religion. In the broadly post-Kantian milieu, nineteenth-century thinkers such as Friedrich Schleiermacher, Albrecht Ritschl, and Adolf von Harnack defined religion essentially, anthropologically, and subjectively. That is, religion has a particular essence, and is in some manner inalienable from our humanity. The emphasis of this conception is on the experience of the religious subject, instead of the knowledge of religion’s object (let alone its reality). It is this notion of religion that both Barth and Bonhoeffer challenged.
(For a[n attempted] summary of the Christian faith, see my essay: “Theology in Outline: What do I Believe?“)
However, despite the challenge they issued to their shared intellectual heritage, Barth and Bonhoeffer appear to diverge on both the definition and, therefore, the critique of religion – at least during the stage of Bonhoeffer’s 1943-45 imprisonment. While Barth unleashed a thoroughgoing theological critique of religion as faithlessness [Unglaube], he also insisted that humans were always and unavoidably religious. Barth maintained that, despite the liabilities of religion, we cannot and should not be religionless because we are not truly godless. Bonhoeffer, however, spoke in 1944-45 of a desirably “religionless Christianity.” This, despite the fact that he ostensibly intended to carry forward Barth’s theological critique of religion – which was, in Bonhoeffer’s opinion, Barth’s “greatest merit” as a theologian.
Whether Barth and Bonhoeffer share a common theological critique of religion has been subject to intense scholarly debate. To answer this question, we need first to ask another: What did Barth and Bonhoeffer mean by the term “religion”? I propose that, although Barth’s and Bonhoeffer’s definitions of religion diverge, their critiques of religion converge. Barth developed a systematic/dialectical concept of religion as self-justification, which the early Bonhoeffer inherited. However, in prison, Bonhoeffer developed a historical/psychological definition of religion as an inward and partial approach to human life. We must realize that these are two different definitions of religion, lest we compare apples to oranges, as it were, and conclude that Barth’s and Bonhoeffer’s critiques of religion also diverged.
Once we realize the divergent definitions, we can see the convergent critiques of a particular essence of religion: the self-justifying projection of a deity – a projection which calls for theological analysis. That is, for both Barth and Bonhoeffer, at the heart of “religion” is the impulse to posit and make room for a “God,” in order to secure our own identities by means of and over against this deity. Although religion, thus understood, is inescapable, it is not constitutive of our humanity.
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 See Christine Axt-Piscalar, “Liberal Theology in Germany,” in The Blackwell Companion to Nineteenth-Century Theology, ed. David Fergusson (Malden, MA: Wiley-Blackwell, 2010), 468–85; Ernst Feil et al., “Religion,” in Religion Past and Present: Encyclopedia of Theology and Religion, vol. 11 (Leiden: Brill, 2012), 31–55; James C. Livingston, Modern Christian Thought: The Enlightenment and the Nineteenth Century, 2nd ed., vol. 1 (Minneapolis: Fortress, 2006).
 See Karl Barth, On Religion: The Revelation of God as the Sublimation of Religion, trans. Garrett Green (London; New York: T&T Clark, 2006). This is a new translation of §17 in Karl Barth, Church Dogmatics, vol. I/2 (Edinburgh: T&T Clark, 1956), 280–361. Henceforth, all references to the Church Dogmatics will appear in the following form: CD I/1, 1.
 CD IV/1, 483.
 Dietrich Bonhoeffer, Letters and Papers from Prison, ed. John W. de Gruchy, trans. Isabel Best et al., DBWE 8 (Minneapolis: Fortress, 2009), 361–67.
 Ibid., 429.