From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
It is right and just, always and for everyone, to serve God, to render him the worship which is his due and to honour one’s parents as they deserve. Positive precepts such as these, which order us to perform certain actions and to cultivate certain dispositions, are universally binding; they are “unchanging”.94 They unite in the same common good all people of every period of history, created for “the same divine calling and destiny”.95 These universal and permanent laws correspond to things known by the practical reason and are applied to particular acts through the judgment of conscience. The acting subject personally assimilates the truth contained in the law. He appropriates this truth of his being and makes it his own by his acts and the corresponding virtues. The negative precepts of the natural law are universally valid. They oblige each and every individual, always and in every circumstance. It is a matter of prohibitions which forbid a given action semper et pro semper, without exception, because the choice of this kind of behaviour is in no case compatible with the goodness of the will of the acting person, with his vocation to life with God and to communion with his neighbour. It is prohibited — to everyone and in every case — to violate these precepts. They oblige everyone, regardless of the cost, never to offend in anyone, beginning with oneself, the personal dignity common to all.
On the other hand, the fact that only the negative commandments oblige always and under all circumstances does not mean that in the moral life prohibitions are more important than the obligation to do good indicated by the positive commandments. The reason is this: the commandment of love of God and neighbour does not have in its dynamic any higher limit, but it does have a lower limit, beneath which the commandment is broken. Furthermore, what must be done in any given situation depends on the circumstances, not all of which can be foreseen; on the other hand there are kinds of behaviour which can never, in any situation, be a proper response — a response which is in conformity with the dignity of the person. Finally, it is always possible that man, as the result of coercion or other circumstances, can be hindered from doing certain good actions; but he can never be hindered from not doing certain actions, especially if he is prepared to die rather than to do evil.
The Church has always taught that one may never choose kinds of behaviour prohibited by the moral commandments expressed in negative form in the Old and New Testaments. As we have seen, Jesus himself reaffirms that these prohibitions allow no exceptions: “If you wish to enter into life, keep the commandments . . . You shall not murder, You shall not commit adultery, You shall not steal, You shall not bear false witness” (Mt 19:17-18).
94 Cf. Second Vatican Ecumenical Council, Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 10; Sacred Congregation for the Doctrine of the Faith, Declaration on Certain Questions Concerning Sexual Ethics Persona Humana (December 29,1975), 4: Acta Apostolicæ Sedis 68 (1976), 80: “But in fact, divine Revelation and, in its own proper order, philosophical wisdom, emphasize the authentic exigencies of human nature. They thereby necessarily manifest the existence of immutable laws inscribed in the constitutive elements of human nature and which are revealed to be identical in all beings endowed with reason.”
95 Second Vatican Ecumenical Council, Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 29.
— Veritatis Splendor 50