In some European countries the ideology of 'Christian Democracy' was an essential instrument through which the cultural and social identities associated with the Catholic Church entered into the world of modern constitutionalism and modern politics. This article explores many of the problems related to the definition of such an ideology: its linkage with the so-called social doctrine of the Catholic Church as well as the differences between them, the long historical journey which culminated in establishing 'democracy' as the unavoidable panorama for modern politics, and the relationship existing between official Vatican thought and the contribution of some Catholic thinkers, such as Maritain, Mounier and Dossetti. The climax of the ideology of Christian Democracy lay in its fundamental contribution to the constitutional documents of the post-1945 period in France, Italy and Germany. That contribution established a model through which a particular version of social democracy was realized. The assimilation of Christian democratic ideology into social democracy also signalled the dissolution of the former and enables us in retrospect to identify its 'transitional' character.