Eduardo Mendieta in The Immanent Frame:
The centrality of religion to social theory in general and philosophy in particular explains why Jürgen Habermas has dealt with it, in both substantive and creative ways, in all of his work. Indeed, religion can be used as a lens through which to glimpse both the coherence and the transformation of his distinctive theories of social development and his rethinking of the philosophy of reason as a theory of social rationalization.
For Habermas, religion has been a continuous concern precisely because it is related to both the emergence of reason and the development of a public space of reason-giving. Religious ideas, according to Habermas, are never mere irrational speculation. Rather, they possess a form, a grammar or syntax, that unleashes rational insights, even arguments; they contain, not just specific semantic contents about God, but also a particular structure that catalyzes rational argumentation.
We could say that in his earliest, anthropological-philosophical stage, Habermas approaches religion from a predominantly philosophical perspective. But as he undertakes the task of “transforming historical materialism” that will culminate in his magnum opus, The Theory of Communicative Action, there is a shift from philosophy to sociology and, more generally, social theory. With this shift, religion is treated, not as a germinal for philosophical concepts, but instead as the source of the social order. This approach is of course shaped by the work of the classics of sociology: Weber, Durkheim, and even Freud. What is noteworthy about this juncture in Habermas’s writings is that secularization is explained as “pressure for rationalization” from “above,” which meets the force of rationalization from below, from the realm of technical and practical action oriented to instrumentalization. Additionally, secularization here is not simply the process of the profanation of the world—that is, the withdrawal of religious perspectives as worldviews and the privatization of belief—but, perhaps most importantly, religion itself becomes the means for the translation and appropriation of the rational impetus released by its secularization. Here, religion becomes its own secular catalyst, or, rather, secularization itself is the result of religion.