From Pope St. John Paul II’s Apostolic Exhortation Reconciliatio et Pænitentia (1984):
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. Man perceives that this disobedience to God destroys the bond that unites him with his life principle: It is a mortal sin, that is, an act which gravely offends God and ends in turning against man himself with a dark and powerful force of destruction.
During the synod assembly some fathers proposed a threefold distinction of sins, classifying them as venial, grave and mortal. This threefold distinction might illustrate the fact that there is a scale of seriousness among grave sins. But it still remains true that the essential and decisive distinction is between sin which destroys charity and sin which does not kill the supernatural life: There is no middle way between life and death.
Likewise, care will have to be taken not to reduce mortal sin to an act of “fundamental option”—as is commonly said today—against God, intending thereby an explicit and formal contempt for God or neighbor. 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. Thus the fundamental orientation can be radically changed by individual acts. Clearly there can occur situations which are very complex and obscure from a psychological viewpoint and which have an influence on the sinner’s subjective culpability. But from a consideration of the psychological sphere one cannot proceed to the construction of a theological category, which is what the “fundamental option” precisely is, understanding it in such a way that it objectively changes or casts doubt upon the traditional concept of mortal sin.
While every sincere and prudent attempt to clarify the psychological and theological mystery of sin is to be valued, the church nevertheless has a duty to remind all scholars in this field of the need to be faithful to the word of God that teaches us also about sin. She likewise has to remind them of the risk of contributing to a further weakening of the sense of sin in the modern world.
— Reconciliatio et Pænitentia 17