This article identifies three different conceptions of legitimation - pre-modern, modern, and post-secular - that compete both within and across national boundaries for the coveted prize of informing the social imaginary regarding how the government and the law should be legitimated in constitutional democracies. Pre-modern conceptions of legitimation consider governments and rulers legitimate if they are ordained by God or if the political system is ordered in accordance with the normative cosmic order. Contemporary proponents of the pre-modern conception range from those in the United States who maintain that the government has been legitimated by the “Judeo-Christian tradition” to those in predominantly Muslim countries like Iran that have constitutional theocracies. By contrast, the prevailing modern conception of legitimation in constitutional democracies stems from the “consent of the governed,” which includes two principles of legitimation - the principle of democracy (or popular sovereignty) and the principle of constitutionalism (or the rule of law). The critical challenges to these principles include the internal challenges of identity politics and religious fundamentalism and the external challenge of globalization. Despite the predominance of the modern conception, there are strong signs that a post-secular conception of legitimation is on the horizon which would transform the prevailing understanding of the principles of democracy and constitutionalism. The dramatic return of religion and the surprising rise of political theology are two prominent developments supporting a shift to a post-secular conception of legitimation and a new post-secular social imaginary.