Simon During in The Immanent Frame:
Is absolute secularity conceivable? The question arises from the paradoxical intuition that the secularization thesis is simultaneously both right and muddled. Perhaps the most fundamental problem with the broader secularization thesis (which I take to claim that, over the past half-millennium or so, Western society has undergone a systemic diminution of religious practice) is that it isn’t clear what the non-secular is. After all, it can be extended from those beliefs and practices that avowedly depend on religious revelation to those that affirm some form of transcendentalism, though they may make no room for God as such. But for a long time both radical atheists and Christian apologists have argued that what looks as if it is secular through and through may not, in fact, be secular at all. From this point of view, important elements of enlightened secularity in particular can be understood, not as Christianity’s overcoming, but as its displacement. Thus, for instance, in his Scholasticism and Politics (1938), Jacques Maritain, following Nietzsche, speaks of the “Christian leaven fermenting in the bosom of human history” as the source of democratic modernity. Here the secular, political concept of human equality is seen to have a Christian origin and to bear a continuing Christian charge, even though its purposes and contexts have changed.
Numerous applications of the displacement model of secularization are current, but here I will point to just one. It concerns philosophical anthropology. The argument is that certain post-Enlightenment concepts of the human (or of “man”) remain Christian in their deep structures. Of these, the most important is the philosophical anthropology of negation (to use Marcel Gauchet’s term), according to which human nature is not just appetitive but necessarily incomplete, that is to say, inadequate to its various ecologies and conditions, and for that reason beset by fear, uneasiness, anxiety, and so on. For those who accept the displacement model, this anthropology, even in its modern forms, remains dependent on the revealed doctrine that human nature as such is fallen. Philosophical anthropology is important for thinking about secularization because the secularization thesis often becomes a proxy for the argument that secularity places human nature at risk.