For thinkers working in this tradition, Augustine and Aquinas preeminent among them, the fundamental philosophical problem was that of contingency—how and why anything exists at all, when it plainly need not. In the course of wrestling with this question, Christian philosophy arrived at its great insight: that contingent beings depend on a God whose very nature simply is to be. Its central theses, clarified over a millennium of philosophic labor, comprised what Étienne Gilson called the “existential” character of Christian theism. They included the demonstration that God does not “have” existence but is himself the pure act of existence; that contingent things are not identical with their existence and are sustained in being by God; and that to know the nature of any finite thing is to know its likeness to its divine cause. These claims, and the conception of rationality embedded within them, provided the metaphysical infrastructure of Catholic Christianity, whose intellectual history is unintelligible apart from it.
Matthew Rose on Charles Taylor