From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
In any event, it is always from the truth that the dignity of conscience derives. In the case of the correct conscience, it is a question of the objective truth received by man; in the case of the erroneous conscience, it is a question of what man, mistakenly, subjectively considers to be true. It is never acceptable to confuse a “subjective” error about moral good with the “objective” truth rationally proposed to man in virtue of his end, or to make the moral value of an act performed with a true and correct conscience equivalent to the moral value of an act performed by following the judgment of an erroneous conscience.108 It is possible that the evil done as the result of invincible ignorance or a non-culpable error of judgment may not be imputable to the agent; but even in this case it does not cease to be an evil, a disorder in relation to the truth about the good. Furthermore, a good act which is not recognized as such does not contribute to the moral growth of the person who performs it; it does not perfect him and it does not help to dispose him for the supreme good. Thus, before feeling easily justified in the name of our conscience, we should reflect on the words of the Psalm: “Who can discern his errors? Clear me from hidden faults” (Ps 19:12). There are faults which we fail to see but which nevertheless remain faults, because we have refused to walk towards the light (cf. Jn 9:39-41).
Conscience, as the ultimate concrete judgment, compromises its dignity when it is culpably erroneous, that is to say, “when man shows little concern for seeking what is true and good, and conscience gradually becomes almost blind from being accustomed to sin”.109 Jesus alludes to the danger of the conscience being deformed when he warns: “The eye is the lamp of the body. So if your eye is sound, your whole body will be full of light; but if your eye is not sound, your whole body will be full of darkness. If then the light in you is darkness, how great is the darkness!” (Mt 6:22-23).
108 Cf. Saint Thomas Aquinas, De Veritate, q. 17, a. 4.
109 Second Vatican Ecumenical Council, Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 16.
— Veritatis Splendor 63