The divide that needs bridging is that contained in Pascal's famous exclamation: "Dieu d'Abraham, Dieu d'Isaac, Dieu de Jacob, non des Philosophes et des savants." "God of Abraham, Isaac and Jacob, not the God of philosophers and scientists."
Pascal, one of our conventional precursors of the existentialist mood, sets the distinction as a choice, and he, a philosopher and scientist of the first rank, chooses the historical God in whom the Patriarchs placed their faith. It is a choice that our contemporary world, for all its deference to science, tends to prefer, at least when the third path of atheism is shunned.
It is not that the God of the philosophers was ever necessarily intended, at least by Jewish and Christian philosophers, to be different from the God of the biblical revelation. But Aristotle's Unmoved mover, Anselm's that-than-which-no-greater-can-be-conceived, Thomas' breaker of infinite chains, Leibniz's chief Monad, Spinoza's universal Substance, Kant's postulate of practical reason--these entities seem far removed from the one for whom the prophets spoke, or the Son of Man.
I have been reading Karl Rahner's Grundkurs des Glaubens, and I am about a quarter of the way through. (Attentive readers will note that, for better or worse, I tend to talk more about books I'm reading than about those I've finished. This is a weakness of mine. I ought to linger more, and reflect back when I finish, but I'm unhappily more inclined to breathlessly run on to something else when that final page is turned.)
Rahner seems to be approaching Christianity through two distinct philosophical streams. He follows Kant in developing the implications of human transcendence--freedom and subjectivity presupposing a realm outside of the categoreal, historical world of objects, things, beings, subject to the categories of space, time, number, modality and causation. God is the horizon, the referent, the ground, the necessary presupposition, whose well of infinite potentiality makes possible the creaturely world we inhabit, the world which we dimly intimate is not the whole of what is.
Rahner also draws on Heidegger, presenting human being, in time, as radically free, and radically threatened by the guilt that necessarily accompanies the responsibility of continually choosing when there is no certainty about which choices one ought to make.
So far, so good. This is another version of the God of the Philosophers, a pretty difficult one, but one responsive and intelligible to our time. How, then, does Rahner, the Catholic, the Jesuit, bridge the obvious gap between the transcendental boundary of what is universally human and the Catholic notion of a God who works in history and, in the fullness of time, became flesh and dwelt among us?
His answer seems to lie in human existence. The transcendent is not in time, and has no history. But human beings, who apprehend the transcendent, do live in time, and our experience of the transcendent itself has a history, which in turn becomes the history of revelation. And it is my guess that it is that unique bridge of human existence, between the transcendent and the historical, that gets its fullest expression in what becomes the most distinctively Christian dogma, the incarnation. We shall see.