This article looks at the contribution of secularisation debates to a critical theory of society. As the relations between the ‘religious’ and ‘secular’ aspects of modern life grow more vexing, it argues critical theory must eschew its previous secularisation-as-progress metanarrative. Instead, processes of secularisation are better understood as those relationships between public and private beliefs and practices that take place at the boundaries between modern society’s commitment to procedural institutions and substantive value commitments. The article then revisits four different understandings of secularisation that, coming from a variety of intellectual traditions, help us redefine it beyond an exclusive focus on institutional religions: normative questions on the rise and decline of autonomous values; temporal questions on the self-positing of modernity as an historical epoch; political questions on the desacralisation of modern sovereignty and practical questions on the non-technological dimensions of technology. This framework is put to the test in relation to the procedural challenge of democratic fallibility and the substantive challenge of planetary survival.