From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
Finally, martyrdom is an outstanding sign of the holiness of the Church. Fidelity to God’s holy law, witnessed to by death, is a solemn proclamation and missionary commitment usque ad sanguinem, so that the splendour of moral truth may be undimmed in the behaviour and thinking of individuals and society. This witness makes an extraordinarily valuable contribution to warding off, in civil society and within the ecclesial communities themselves, a headlong plunge into the most dangerous crisis which can afflict man: the confusion between good and evil, which makes it impossible to build up and to preserve the moral order of individuals and communities. By their eloquent and attractive example of a life completely transfigured by the splendour of moral truth, the martyrs and, in general, all the Church’s Saints, light up every period of history by reawakening its moral sense. By witnessing fully to the good, they are a living reproof to those who transgress the law (cf. Wis 2:12), and they make the words of the Prophet echo ever afresh: “Woe to those who call evil good and good evil, who put darkness for light and light for darkness, who put bitter for sweet and sweet for bitter!” (Is 5:20).
Although martyrdom represents the high point of the witness to moral truth, and one to which relatively few people are called, there is nonetheless a consistent witness which all Christians must daily be ready to make, even at the cost of suffering and grave sacrifice. Indeed, faced with the many difficulties which fidelity to the moral order can demand, even in the most ordinary circumstances, the Christian is called, with the grace of God invoked in prayer, to a sometimes heroic commitment. In this he or she is sustained by the virtue of fortitude, whereby — as Gregory the Great teaches — one can actually “love the difficulties of this world for the sake of eternal rewards”.146
146 Moralia in Iob, VII, 21, 24: Patrologiæ Cursus completus, Series Latina 75, 778: “huius mundi aspera pro æternis præmiis amore.”
— Veritatis Splendor 93