From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
Conscience, as the judgment of an act, is not exempt from the possibility of error. As the [Second Vatican] Council puts it, “not infrequently conscience can be mistaken as a result of invincible ignorance, although it does not on that account forfeit its dignity; but this cannot be said when a man shows little concern for seeking what is true and good, and conscience gradually becomes almost blind from being accustomed to sin”.107 In these brief words the Council sums up the doctrine which the Church down the centuries has developed with regard to the erroneous conscience.
Certainly, in order to have a “good conscience” (1 Tim 1:5), man must seek the truth and must make judgments in accordance with that same truth. As the Apostle Paul says, the conscience must be “confirmed by the Holy Spirit” (cf. Rom 9:1); it must be “clear” (2 Tim 1:3); it must not “practise cunning and tamper with God’s word”, but “openly state the truth” (cf. 2 Cor 4:2). On the other hand, the Apostle also warns Christians: “Do not be conformed to this world but be transformed by the renewal of your mind, that you may prove what is the will of God, what is good and acceptable and perfect” (Rom 12:2).
Paul’s admonition urges us to be watchful, warning us that in the judgments of our conscience the possibility of error is always present. Conscience is not an infallible judge; it can make mistakes. However, error of conscience can be the result of an invincible ignorance, an ignorance of which the subject is not aware and which he is unable to overcome by himself.
The Council reminds us that in cases where such invincible ignorance is not culpable, conscience does not lose its dignity, because even when it directs us to act in a way not in conformity with the objective moral order, it continues to speak in the name of that truth about the good which the subject is called to seek sincerely.
107 Second Vatican Ecumenical Council, Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 16.
— Veritatis Splendor 62