23 As far as the two great themes of “reason” and “freedom” are concerned, here we can only touch upon the issues connected with them. Yes indeed, reason is God's great gift to man, and the victory of reason over unreason is also a goal of the Christian life. But when does reason truly triumph? When it is detached from God? When it has become blind to God? Is the reason behind action and capacity for action the whole of reason? If progress, in order to be progress, needs moral growth on the part of humanity, then the reason behind action and capacity for action is likewise urgently in need of integration through reason's openness to the saving forces of faith, to the differentiation between good and evil. Only thus does reason become truly human. It becomes human only if it is capable of directing the will along the right path, and it is capable of this only if it looks beyond itself. Otherwise, man's situation, in view of the imbalance between his material capacity and the lack of judgement in his heart, becomes a threat for him and for creation. Thus where freedom is concerned, we must remember that human freedom always requires a convergence of various freedoms. Yet this convergence cannot succeed unless it is determined by a common intrinsic criterion of measurement, which is the foundation and goal of our freedom. Let us put it very simply: man needs God, otherwise he remains without hope. Given the developments of the modern age, the quotation from Saint Paul with which I began ( Eph 2:12) proves to be thoroughly realistic and plainly true. There is no doubt, therefore, that a “Kingdom of God” accomplished without God—a kingdom therefore of man alone—inevitably ends up as the “perverse end” of all things as described by Kant: we have seen it, and we see it over and over again. Yet neither is there any doubt that God truly enters into human affairs only when, rather than being present merely in our thinking, he himself comes towards us and speaks to us. Reason therefore needs faith if it is to be completely itself: reason and faith need one another in order to fulfil their true nature and their mission.
Source: Spe Salvi (Vatican.va)