70 The Post-Synodal Apostolic Exhortation Reconciliatio et Paenitentia reaffirmed the importance and permanent validity of the distinction between mortal and venial sins, in accordance with the Church's tradition. And the 1983 Synod of Bishops, from which that Exhortation emerged, "not only reaffirmed the teaching of the Council of Trent concerning the existence and nature of mortal and venial sins, but it also recalled that mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent". The statement of the Council of Trent does not only consider the "grave matter" of mortal sin; it also recalls that its necessary condition is "full awareness and deliberate consent". In any event, both in moral theology and in pastoral practice one is familiar with cases in which an act which is grave by reason of its matter does not constitute a mortal sin because of a lack of full awareness or deliberate consent on the part of the person performing it. Even so, "care will have to be taken not to reduce mortal sin to an act of 'fundamental option' — as is commonly said today — against God", seen either as an explicit and formal rejection of God and neighbour or as an implicit and unconscious rejection of love. "For mortal sin exists also when a person knowingly and willingly, for whatever reason, chooses something gravely disordered. In fact, such a choice already includes contempt for the divine law, a rejection of God's love for humanity and the whole of creation: the person turns away from God and loses charity. Consequently, the fundamental orientation can be radically changed by particular acts. Clearly, situations can occur which are very complex and obscure from a psychological viewpoint, and which influence the sinner's subjective imputability. But from a consideration of the psychological sphere one cannot proceed to create a theological category, which is precisely what the 'fundamental option' is, understanding it in such a way that it objectively changes or casts doubt upon the traditional concept of mortal sin". The separation of fundamental option from deliberate choices of particular kinds of behaviour, disordered in themselves or in their circumstances, which would not engage that option, thus involves a denial of Catholic doctrine on mortal sin: "With the whole tradition of the Church, we call mortal sin the act by which man freely and consciously rejects God, his law, the covenant of love that God offers, preferring to turn in on himself or to some created and finite reality, something contrary to the divine will ( conversio ad creaturam ). This can occur in a direct and formal way, in the sins of idolatry, apostasy and atheism; or in an equivalent way, as in every act of disobedience to God's commandments in a grave matter".
Source: Veritatis Splendor (Vatican.va)