From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
The modern concern for the claims of autonomy has not failed to exercise an influence also in the sphere of Catholic moral theology. While the latter has certainly never attempted to set human freedom against the divine law or to question the existence of an ultimate religious foundation for moral norms, it has, nonetheless, been led to undertake a profound rethinking about the role of reason and of faith in identifying moral norms with reference to specific “innerworldly” kinds of behaviour involving oneself, others and the material world.
It must be acknowledged that underlying this work of rethinking there are certain positive concerns which to a great extent belong to the best tradition of Catholic thought. In response to the encouragement of the Second Vatican Council,60 there has been a desire to foster dialogue with modern culture, emphasizing the rational — and thus universally understandable and communicable — character of moral norms belonging to the sphere of the natural moral law.61 There has also been an attempt to reaffirm the interior character of the ethical requirements deriving from that law, requirements which create an obligation for the will only because such an obligation was previously acknowledged by human reason and, concretely, by personal conscience.
Some people, however, disregarding the dependence of human reason on Divine Wisdom and the need, given the present state of fallen nature, for Divine Revelation as an effective means for knowing moral truths, even those of the natural order,62 have actually posited a complete sovereignty of reason in the domain of moral norms regarding the right ordering of life in this world. Such norms would constitute the boundaries for a merely “human” morality; they would be the expression of a law which man in an autonomous manner lays down for himself and which has its source exclusively in human reason. In no way could God be considered the Author of this law, except in the sense that human reason exercises its autonomy in setting down laws by virtue of a primordial and total mandate given to man by God. These trends of thought have led to a denial, in opposition to Sacred Scripture (cf. Mt 15:3-6) and the Church’s constant teaching, of the fact that the natural moral law has God as its author, and that man, by the use of reason, participates in the eternal law, which it is not for him to establish.
60 Cf. Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 40 and 43.
61 Cf. Saint Thomas Aquinas, Summa Theologica, I-II, q. 71, a. 6; see also ad Sum.
62 Cf. Pius XII, Encyclical Letter Humani Generis (August 12,1950): Acta Apostolicæ Sedis 42 (1950), 561-562.
— Veritatis Splendor 36