From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
The importance of this interior dialogue of man with himself can never be adequately appreciated. But it is also a dialogue of man with God, the author of the law, the primordial image and final end of man. Saint Bonaventure teaches that “conscience is like God’s herald and messenger; it does not command things on its own authority, but commands them as coming from God’s authority, like a herald when he proclaims the edict of the king. This is why conscience has binding force”.103 Thus it can be said that conscience bears witness to man’s own rectitude or iniquity to man himself but, together with this and indeed even beforehand, conscience is the witness of God himself, whose voice and judgment penetrate the depths of man’s soul, calling him fortiter et suaviter to obedience. “Moral conscience does not close man within an insurmountable and impenetrable solitude, but opens him to the call, to the voice of God. In this, and not in anything else, lies the entire mystery and the dignity of the moral conscience: in being the place, the sacred place where God speaks to man”.104
103 In II Librum Sentent., dist. 39, a. 1, q. 3, conclusion; Ed. Ad Claras Aquas, II 907b.
104 Address (General Audience, August 17, 1983), 2: Insegnamenti, VI, 2 (1983), 256.
— Veritatis Splendor 58