How to theorise religion in International Relations (IR)? Does the concept of post-secularity advance the debate on religion beyond the ‘return of religion’ and the crisis of secular reason? This article argues that the post-secular remains trapped in the logic of secularism. First, a new account is provided of the ‘secularist bias’ that characterises mainstream IR theory: (a) defining religion in either essentialist or epiphenomenal terms; (b) positing a series of ‘antagonistic binary opposites’ such as the secular versus the religious; (...) (c) de-sacralising and re-sacralising the public square. The article then analyses post-secularity, showing that it subordinates faith under secular reason and sacralises the ‘other’ by elevating difference into the sole transcendental term. Theorists of the post-secular such as Jürgen Habermas or William Connolly also equate secular modernity with metaphysical universalism, which they seek to replace with post-metaphysical pluralism. In contrast, the alternative that this article outlines is an international theory that develops the Christian realism of the English School in the direction of a metaphysical-political realism. Such a realism binds together reason with faith and envisions a ‘corporate’ association of peoples and nations beyond the secularist settlement of Westphalia that is centred on national states and transnational markets. By linking immanent values to transcendent principles, this approach can rethink religion in international affairs and help revive grand theory in IR. (shrink)