29 It is on faith in God, preserved pure and stainless, that man's morality is based. All efforts to remove from under morality and the moral order the granite foundation of faith and to substitute for it the shifting sands of human regulations, sooner or later lead these individuals or societies to moral degradation. The fool who has said in his heart "there is no God" goes straight to moral corruption ( Psalms xiii. 1), and the number of these fools who today are out to sever morality from religion, is legion. They either do not see or refuse to see that the banishment of confessional Christianity, i.e., the clear and precise notion of Christianity, from teaching and education, from the organization of social and political life, spells spiritual spoliation and degradation. No coercive power of the State, no purely human ideal, however noble and lofty it be, will ever be able to make shift of the supreme and decisive impulses generated by faith in God and Christ. If the man, who is called to the hard sacrifice of his own ego to the common good, loses the support of the eternal and the divine, that comforting and consoling faith in a God who rewards all good and punishes all evil, then the result of the majority will be, not the acceptance, but the refusal of their duty. The conscientious observation of the ten commandments of God and the precepts of the Church (which are nothing but practical specifications of rules of the Gospels) is for every one an unrivaled school of personal discipline, moral education and formation of character, a school that is exacting, but not to excess. A merciful God, who as Legislator, says - Thou must! - also gives by His grace the power to will and to do. To let forces of moral formation of such efficacy lie fallow, or to exclude them positively from public education, would spell religious under-feeding of a nation. To hand over the moral law to man's subjective opinion, which changes with the times, instead of anchoring it in the holy will of the eternal God and His commandments, is to open wide every door to the forces of destruction. The resulting dereliction of the eternal principles of an objective morality, which educates conscience and ennobles every department and organization of life, is a sin against the destiny of a nation, a sin whose bitter fruit will poison future generations.
Source: Mit Brennender Sorge (Vatican.va)