German Idealism ran for roughly sixty years after Immanuel Kant's critical philosophy reshaped what a system of philosophy could try to do. Kant had argued that the mind imposes categories of space, time and causality onto experience rather than reading them off a world it perceives directly, which left the deepest structure of reality permanently beyond reach. His successors read that limitation as an invitation rather than a wall. Johann Gottlieb Fichte tried to derive the whole of experience from the activity of a self positing itself, Friedrich Wilhelm Joseph Schelling looked for the same activity within nature itself, and Georg Wilhelm Friedrich Hegel built the most systematic and most influential answer of the three, in which an evolving Absolute Spirit comes to full self consciousness through the unfolding of logic, nature and human history together.
Hegel's system treated Christian doctrine as one of philosophy's most important case studies rather than a subject to set aside. He read the Trinity as a picture, in figurative form, of Spirit's own three stage movement of unity, division and reconciliation, and he read the Incarnation as the doctrine that finite and infinite are not, after all, permanently opposed. Liberal Protestant theology in the following century absorbed a great deal of this vocabulary even where it rejected Hegel's system outright, and conservative theologians spent just as much energy resisting it, which is its own kind of influence. Few philosophical movements have argued so directly, in either direction, over what Christian doctrine actually means.