Early modern social actors made a distinction between spiritual institutions and temporal institutions. The distinction was crucial for tolerance rhetoric—philosophers like John Locke argued that spiritual institutions were properly outside the jurisdiction of temporal institutions, such as the state.
Such a distinction makes sense only if we assume a supernaturalist ontology, according to which there are inter-human relations, and then there are relations with God, “the divine,” “the sacred” or something like that. The argument is homologous to the following contemporary slogan:
If we assume a materialist ontology, however, “temporal” institutions are all we have. On such a view, the distinction between spiritual and temporal institutions lacks all analytical usefulness.
If that’s the case, is a materialist theory of secularism (or its opposite: the claim that “religion” persists) even possible?
Craig Martin 
This is exactly the point that many scholars who work on secularism miss, thereby smuggling in the supernatural ontology they think they’re historicizing. There is a nostalgia, I find, whereby they historicize the secular but naturalize the religious–akin to Eliade’s notion of the once homogenously religious world being disrupted by the Enlightenment’s rationalism and thus the secular. The only way to proceed, I’d argue, is to see sacred/secular as nothing but a mundane rhetorical pairing, not dissimilar to pure/impure, and then study the way the pairs are defined in light of each other and how they are used–and the types of social worlds that the pairing makes possible when used this or that way,
Yep.You argue that well in The Sacred Is the Profane. 😉