From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
The great concern of our contemporaries for historicity and for culture has led some to call into question the immutability of the natural law itself, and thus the existence of “objective norms of morality”96 valid for all people of the present and the future, as for those of the past. Is it ever possible, they ask, to consider as universally valid and always binding certain rational determinations established in the past, when no one knew the progress humanity would make in the future?
It must certainly be admitted that man always exists in a particular culture, but it must also be admitted that man is not exhaustively defined by that same culture. Moreover, the very progress of cultures demonstrates that there is something in man which transcends those cultures. This “something” is precisely human nature: this nature is itself the measure of culture and the condition ensuring that man does not become the prisoner of any of his cultures, but asserts his personal dignity by living in accordance with the profound truth of his being. To call into question the permanent structural elements of man which are connected with his own bodily dimension would not only conflict with common experience, but would render meaningless Jesus’ reference to the “beginning”, precisely where the social and cultural context of the time had distorted the primordial meaning and the role of certain moral norms (cf. Mt 19:1-9). This is the reason why “the Church affirms that underlying so many changes there are some things which do not change and are ultimately founded upon Christ, who is the same yesterday and today and for ever”.97 Christ is the “Beginning” who, having taken on human nature, definitively illumines it in its constitutive elements and in its dynamism of charity towards God and neighbour.98
Certainly there is a need to seek out and to discover the most adequate formulation for universal and permanent moral norms in the light of different cultural contexts, a formulation most capable of ceaselessly expressing their historical relevance, of making them understood and of authentically interpreting their truth. This truth of the moral law — like that of the “deposit of faith” — unfolds down the centuries: the norms expressing that truth remain valid in their substance, but must be specified and determined “eodem sensu eademque sententia“99 in the light of historical circumstances by the Church’s Magisterium, whose decision is preceded and accompanied by the work of interpretation and formulation characteristic of the reason of individual believers and of theological reflection.100
96 Cf. Second Vatican Ecumenical Council, Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 16.
97 Ibid., 10.
98 Cf. Saint Thomas Aquinas, Summa Theologiæ I-II, q.108, a. 1. St. Thomas bases the fact that moral norms, even in the context of the New Law, are not merely formal in character but have a determined content, upon the assumption of human nature by the Word.
99 Saint Vincent of Lerins, Commonitorium Primum, c. 23: Patrologiæ Cursus completus, Series Græca 50, 668.
100 The development of the Church’s moral doctrine is similar to that of the doctrine of the faith (cf. First Vatican Ecumenical Council, Dogmatic Constitution on the Catholic Faith Dei Filius, Chap. 4: H. Denzinger-A Schönmetzer, Enchiridion Symbolorum Definitionum et Declarationum de Rebus Fidei et Morum [DS], 3020, and Canon 4: DS, 3024). The words spoken by John XXIII at the opening of the Second Vatican Council can also be applied to moral doctrine: “This certain and unchanging teaching (i.e., Christian doctrine in its completeness), to which the faithful owe obedience, needs to be more deeply understood and set forth in a way adapted to the needs of our time. Indeed, this deposit of the faith, the truths contained in our time-honored teaching, is one thing; the manner in which these truths are set forth (with their meaning preserved intact) is something else”: Acta Apostolicæ Sedis 54 (1962), 792; cf. L’Osservatore Romano, October 12, 1962, p. 2.
— Veritatis Splendor 53