3 There is then, Venerable Brethren, no reason for wonder that the corruption of morals and depravity of life is already so great, and ever increasingly greater, not only among uncivilized peoples but even in those very nations that are called Christian. The Apostle Paul, writing to the Ephesians, repeatedly admonished them in these words: "But immorality and every uncleanness or covetousness, let it not even be named among you, as become saints; or obscenity or foolish talk." He also places the foundation of holiness and sound morals upon a knowledge of divine things - which holds in check evil desires: "See to it therefore, brethren, that you walk with care: not as unwise but as wise. . . Therefore, do not become foolish, but understand what the will of the Lord is." And rightly so. For the will of man retains but little of that divinely implanted love of virtue and righteousness by which it was, as it were, attracted strongly toward the real and not merely apparent good. Disordered by the stain of the first sin, and almost forgetful of God, its Author, it improperly turns every affection to a love of vanity and deceit. This erring will, blinded by its own evil desires, has need therefore of a guide to lead it back to the paths of justice whence it has so unfortunately strayed. The intellect itself is this guide, which need not be sought elsewhere, but is provided by nature itself. It is a guide, though, that, if it lack its companion light, the knowledge of divine things, will be only an instance of the blind leading the blind so that both will fall into the pit. The holy king David, praising God for the light of truth with which He had illumined the intellect, exclaimed: "The light of Thy countenance, O Lord, is signed upon us." Then he described the effect of this light by adding: "Thou hast given gladness in my heart," gladness, that is, which enlarges our heart so that it runs in the way of God's Commandments.
Source: Acerbo Nimis (Vatican.va)