From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
Like the natural law itself and all practical knowledge, the judgment of conscience also has an imperative character: man must act in accordance with it. If man acts against this judgment or, in a case where he lacks certainty about the rightness and goodness of a determined act, still performs that act, he stands condemned by his own conscience, the proximate norm of personal morality. The dignity of this rational forum and the authority of its voice and judgments derive from the truth about moral good and evil, which it is called to listen to and to express. This truth is indicated by the “divine law”, the universal and objective norm of morality. The judgment of conscience does not establish the law; rather it bears witness to the authority of the natural law and of the practical reason with reference to the supreme good, whose attractiveness the human person perceives and whose commandments he accepts. “Conscience is not an independent and exclusive capacity to decide what is good and what is evil. Rather there is profoundly imprinted upon it a principle of obedience vis-à-vis the objective norm which establishes and conditions the correspondence of its decisions with the commands and prohibitions which are at the basis of human behaviour”.106
106 Encyclical Letter Dominum et Vivificantem (May 18, 1986), 43: Acta Apostolicæ Sedis 78 (1986), 859; cf. Second Vatican Ecumenical Council, Pastoral Constitution on the Church in the Modern World Gaudium et Spes, 16; Declaration on Religious Freedom Dignitatis Humanæ, 3.
— Veritatis Splendor 60