When Divine grace is separated from its reality in God, when grace becomes a thing, a substance, a quality infused into the accidents of humanity, it is only one small step removed from being integrated into the essence of what it means to be human. If this step is taken, and it has been in the ‘modern-turn,’ the turn-to-the-subject, the Gifter of grace no 
longer remains necessary, in a transcendent sense, as grace becomes materialized, immanentized, horizontalized into an ‘immanent frame,’ as Charles Taylor grammarizes. Indeed, Taylor writes with reference to what it means to be human in a frame wherein grace has become the possession, the generative reality of what it means to be a self-determined, self-constructed modern person in the 21st century:
There is another facet of this narrative of secularity which it is worth mentioning here, because of its ubiquity and importance in the “closed” spin on immanence. The story line here is this: once human beings took their norms, their goods, their standards of ultimate value from an authority outside of themselves; from God, or the gods, or the nature of Being or the cosmos. But then they came to see that these higher authorities were their own fictions, and they realized that they had to establish their norms and values for themselves, on their own authority. This is a radicalization of the coming to adulthood story as it figures in the science-driven argument for materialism. It is not just that freed from illusion, humans come to establish true facts about the world. It is also that they come to dictate the ultimate values by which they live.1
Once the secular person came to imagine, through their new ‘social imaginary’ (cf. Taylor) that in fact the classical God was really just a projection of their own imagination, an inner to outer extrapolation of their best selves onto a cipher by which they might live and adjudicate life, they were able to bring diremption for themselves and determine that in fact they were God and thus gods after all. What secularism ultimately brings, this ostensible ‘come of age’ moment, is really just another expression of polytheism, a serpentine belief that humanity itself possesses godness, and thus are the creators of their own reality and existence (in a world where existence and essence have become a singular reality).
As I suggested at the beginning there is a theological origin story behind the secular-turn. Ironically, this turn has a Christian source, albeit as that source has been dressed down by a synthesis of Christian theology with speculative philosophical categories; such categories derived from the classical philosophers like Aristotle et al. The abstraction of grace from the giver or reality of grace, God, takes form most notably in the Catholic theology of Thomas Aquinas. Here we finally get a codification of a burgeoning philosophical frame baptized in the Holy water of the Church. It is the notion of ‘quality’ that takes decisive stage, or in fact ‘substance’ within an Aristotelian frame whereby Divine grace comes to lose its Divine character, at least in the sense that its reality is necessarily grounded in the triune being of Father, Son, and Holy Spirit. Once this move is made, as I asserted earlier, we are only a few small steps, a few small centuries away from ‘coming of age,’ among other contributing factors during and post-Enlightenment. But I find this abstraction of grace from the Divine life to be an interesting development towards modernity, ironically, even as that is given dye within the early mediaeval and Catholic sitz im leben of Thomas Aquinas (and those who received him, whether Catholic or Protestant Reformational). Helmut Thielicke describes the scenario this way:
We can see clearly at this point that what takes place in man, and not merely what happens to him, has become the object of theological observation. It is again evident, as was clear already with respect to the concept of the imago Dei in man’s original state, that the ontic element in the human ego pushes itself into the foreground. Then the theme of theology is not just the relation between God and man; on the contrary, theology then includes as an independent concern a treatment of “anthropology.” Here is where the fault lies.
The crack, or better, the cracks are themselves produced by the belt of tension which necessarily arises where men attempt to combine ontological and personalistic thinking. The greatest strain and the most evident rupture are undoubtedly to be found at the place where grace ceases to be a divine attribute and becomes an effect distinct from the divine attribute from which it emanates, ie, where grace ceases to be a personal relation to man (the “gracious God”) and becomes something which is ontically infused into man and which is thus present in man, demonstrably present. For it is precisely this distinction between the gracious God and the grace given [gratia data] which is the starting point of the distinctively “Roman” development of the doctrine of grace. To put it epigrammatically and therefore with tongue in cheek, what men want is not primarily God himself, but “divine powers which may become human virtues and qualities” (von Harnack). At this point where grace “visibly” passes into man in accordance with certain well defined practices, eg, sacramental operation, it ceases to be exclusively a subject and becomes a material object, “medicine.” This materialization expresses itself in a variety of ways . . . .2
It would surely be reductionistic to blame Thomas and Aristotle for the modern-turn to the self, and what Taylor identifies as ‘the coming of adulthood story.’ That is not my intention. I am simply noticing a Christian turn made at least within the lifetime of early mediaeval developments, that can plausibly help explain how this ‘enlightened-turn’ finally came to fruition. There are many other contributing ideational and socio-culture pressures that finally brought this turn to consummation, but I think it is notable that we already see these fault lines developing as far back as the Hellenic period of the classical philosophers; and then developed more Christianly with the arrival of Thomas and the Romans.
I am only minimally attempting to illustrate how secular ideas can be traced back to a Christian lineage. Charles Taylor, Michael Gillespie, my personal friend, Derrick Peterson, among others have done further work, more substantially, to demonstrate that my point is not ill founded. Christians have as much to do with the secularity of society as anyone else. Indeed, we might go so far as to say that what it means to be a secular-atheist in the 21st century is really just an expression of a Christian heresy that has attended the Church catholic by way of various expressions; whether those expressions be understood within the Church proper, or in society-at-large as the theater of humanity’s glory, albeit, devoid of the Spirit. Ultimately, when grace is abstracted from God and made a quality capable of being understood purely from the immanent frame of the ‘flatlander,’ it is at this point that Genesis 3 once again receives breath to breathe and make another attempt to elevate itself with zigguartic flare to the ‘high places’ of the living God.
1 Charles Taylor, A Secular Age, 580 kindle.
2 Helmut Thielicke, Theological Ethics: Foundations (Philadelphia: Fortress Press, 1966), 238.