The article re-examines Karl Barth's claim that his 1931 book on Anselm of Canterbury's proof of the existence of God from Proslogion 2–4, the Fides Quaerens Intellectum, provides a ‘vital key’ for understanding the rationality of his later theology. The article takes up the question of what ‘dialectics’ denotes in Barth's theology. Re-engaging the issue of dialectics in Karl Barth's theology, particularly in light of the developments of his thought, leads to a critical revision of Bruce McCormack's influential interpretation of Barth's dialectical theology.

Through a close reading of the section, ‘The manner of theology’, from the first of the two parts of Fides Quaerens Intellectum the article sheds light on how Barth envisions the construction of the theoretical concepts of theology in light of his concrete exegetical praxis. By focusing on the relationship between theory and praxis, an understanding of Barth's dialectical theology as a form of ‘speculative reading’ emerges. This notion of dialectics as developing in light of a practice of speculative reading allows us to compare certain aspects of concept-formation in the theology of Karl Barth and the dialectical philosophy of the German Idealist, G. W. F. Hegel. Drawing explicitly on Hegel's analysis of infinite judgement from the second volume of his 1812 Science of Logic, Barth's use of the tautology ‘God is God’ in his second commentary on Romans from 1922 is shown to provide important insight into the structure of Barth's dialectical thinking. This explicitly relates to the important issue of the relationship between faith and reason in Karl Barth's theology, which sheds a new light on how his understanding of the rational basis of the theory and praxis of theology developed from his second Romans commentary onwards.

Finally, Barth's use of dialectics and his account of the rationality of theology as a form of ‘speculative reading’ permits some final reflections on the significance of pneumatology for constructive theology by underscoring how Barth insists on the priority of praxis over theory. To understand Barth's theology and the way it changes over the years is to understand the rationality of its concrete praxis.