In the course of history the relation of Catholicism and Protestantism undergoes, as it were, physiognomical transformations, which, while they do not affect the inmost dogmatic division, strike deep enough to impress on these two systems new and unforeseeable characteristics in different times and places. The essential division in belief, as a theological phenomenon, remains just what it is; and in view of this, it is impossible—even for men of good will—to explain it away or compromise on it. This undebatable and irreducible phenomenon can only be effaced by unity in the Faith. We do not wish prim, arily to treat of the relation of Catholicism and Protestantism from this purely theological aspect of dogmatic difference and reunion, although even our present consideration can only be read under this explicit proviso, and in view of this final and deepest problem. For if Catholicism and Protestantism are from one point of view and in modern history simply two great historical forces, we cannot on that account act as though the problem belonged only to the sphere of historical research into cultural spirituality. Beyond the scope of history there is an enquiry of theological import.
We take as our starting point the fact that Catholicism and Protestantism appear on the stage of history as two forces, each of which presents a whole system of human cultural values, and so an entire totality of life. Ultimately, in the theological field, Catholicism and Protestantism are incommensurable—though even here some qualification is required. But as formative powers of history, as complete forms and formulas of historical existence, Catholicism and Protestantism are fundamentally commensurable. On this plane, then, we find a series of problems which is common to both. Even their very opposition continually creates a common point of contact for their opposing principles. We find, on this plane, a relation between Catholicism and Protestantism which has proved historically to be manageable and valuable, and which at the present time is well defined—a relation sometimes of enmity, sometimes of cold indifference, sometimes of an inner human nearness and readiness to understand.