The issue of legitimation of the constitutional and political order is rapidly increasing in saliency globally, and that includes the West. Mark C. Modak-Truran distinguishes three types of such legitimation: pre-modern, modern and post-secular.
The pre-modern type rests on either a transcendent foundation of the constitutional and political order or the idea of an otherwise pre-existing normative order. Contemporary examples of states legitimized in a (predominantly) pre-modern way are constitutional theocracies such as Iran and Iraq.
The modern type of legitimation is what we in the West, under the influence of rationalization, are most familiar with. According to this conception, the notions of constitutionalism and democracy as such provide sufficient justification for the state. Positive law is regarded as authoritative as long as the legality principle is adhered to. No further questions about the deeper foundations of the notions of constitutionalism and democracy are asked.
In our post-secular age these questions can arguably no longer be avoided, however. The 'new religious pluralism' requires legitimation of a different kind than 'legal fideism', be it primarily at the level of individual citizens.

It is so far not entirely clear whether the ethos of religious pluralism thus required for this third type of legitimation will prove to be strong enough to actually sustain a constitutional and political order. Neither can we be certain that the notions of constitutionalism and democracy will remain in place, or that their contents will essentially remain the same.
It may therefore be of interest to recall that in practice modern conceptions of legitimation often also contain a pre-modern element. Thus, in the West classical liberalism was intimately linked to Christian theological notions such as original sin, the separation of church and state, conscience protection, and the fundamental equality of all human beings before God. One only has to look at the Federalist Papers.
To the extent that a flourishing civil society forms a necessary pre-condition for a well-functioning constitutional democracy, liberalism needs God as well. As the Pope's recent speech to the European Parliament illustrated once again, Christianity in a distinctive manner emphasizes the role of intermediate institutions between the individual and the state, such as the family and voluntary associations.

Could it be the case that the modern type of legitimation, complemented by a touch of pre-modern legitimation, suffices in a post-secular context as well? No convincing alternatives for the notions of constitutionalism and democracy have yet emerged. Paradoxically, moreover, their historically tested transcendent foundations might well prove more inclusive than an artificial, post-secular construct.
To the extent that our post-secular age makes us more aware of the pre-modern foundations of our constitutional and political orders, the new religious pluralism can be welcomed. The resulting questions in terms of legitimation can, however, best be addressed by (re)adopting a classical liberal approach.