From Pope St. John Paul II’s encyclical Veritatis Splendor (1993):
As we have just seen, reflection on the fundamental option has also led some theologians to undertake a basic revision of the traditional distinction between mortal sins and venial sins. They insist that the opposition to God’s law which causes the loss of sanctifying grace — and eternal damnation, when one dies in such a state of sin — could only be the result of an act which engages the person in his totality: in other words, an act of fundamental option. According to these theologians, mortal sin, which separates man from God, only exists in the rejection of God, carried out at a level of freedom which is neither to be identified with an act of choice nor capable of becoming the object of conscious awareness. Consequently, they go on to say, it is difficult, at least psychologically, to accept the fact that a Christian, who wishes to remain united to Jesus Christ and to his Church, could so easily and repeatedly commit mortal sins, as the “matter” itself of his actions would sometimes indicate. Likewise, it would be hard to accept that man is able, in a brief lapse of time, to sever radically the bond of communion with God and afterwards be converted to him by sincere repentance. The gravity of sin, they maintain, ought to be measured by the degree of engagement of the freedom of the person performing an act, rather than by the matter of that act.
— Veritatis Splendor 69