This article sketches the struggles over and the shifting role of Catholicism in the Spanish body politic. It begins by providing a brief overview of the deep historical ties between Catholicism and Spanish identity. It continues by recounting the dialectical process through which a serious social cleavage on the role of religion in politics emerged and percolated over the course of the nineteenth and early twentieth centuries. This cleavage ultimately pit a militant and reactionary brand of authoritarian Catholicism on the right against an equally militant group of secularist ideologues associated with both bourgeois-republican and revolutionary working class (mainly anarchist) political forces. Following Juan Linz, the article emphasizes the nefarious role played by constitution-makers who pursued a partisan secularizing agenda on questions of Church and state in the breakdown of democracy and tragic onset of Civil War. It then delineates the ideology and institutionalization of “national-Catholicism” under Franco, before turning to contrast republican-era constitution-making dynamics with those of the transition to democracy following Franco’s death. It concludes with a discussion of the content of post-transition conflicts over religion and politics, highlighting the constitutional resources for coping with the somehow new yet very old challenge posed by the presence of Islam.