From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
The truth about moral good, as that truth is declared in the law of reason, is practically and concretely recognized by the judgment of conscience, which leads one to take responsibility for the good or the evil one has done. If man does evil, the just judgment of his conscience remains within him as a witness to the universal truth of the good, as well as to the malice of his particular choice. But the verdict of conscience remains in him also as a pledge of hope and mercy: while bearing witness to the evil he has done, it also reminds him of his need, with the help of God’s grace, to ask forgiveness, to do good and to cultivate virtue constantly.
Consequently in the practical judgment of conscience, which imposes on the person the obligation to perform a given act, the link between freedom and truth is made manifest. Precisely for this reason conscience expresses itself in acts of “judgment” which reflect the truth about the good, and not in arbitrary “decisions”. The maturity and responsibility of these judgments — and, when all is said and done, of the individual who is their subject — are not measured by the liberation of the conscience from objective truth, in favour of an alleged autonomy in personal decisions, but, on the contrary, by an insistent search for truth and by allowing oneself to be guided by that truth in one’s actions.
— Veritatis Splendor 61