From Pope St. John Paul II’s Apostolic Exhortation Reconciliatio et Pænitentia (1984):
Furthermore, when sin is considered from the point of view of the punishment it merits, for St. Thomas and other doctors mortal sin is the sin which, if unforgiven, leads to eternal punishment; whereas venial sin is the sin that merits merely temporal punishment (that is, a partial punishment which can be expiated on earth or in purgatory).
Considering sin from the point of view of its matter, the ideas of death, of radical rupture with God, the supreme good, of deviation from the path that leads to God or interruption of the journey toward him (which are all ways of defining mortal sin) are linked with the idea of the gravity of sin’s objective content. Hence, in the church’s doctrine and pastoral action, grave sin is in practice identified with mortal sin.
Here we have the core of the church’s traditional teaching, which was reiterated frequently and vigorously during the recent synod. The synod in fact not only reaffirmed the teaching of the Council of Trent concerning the existence and nature of mortal and venial sins,95 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. It must be added—as was likewise done at the synod—that some sins are intrinsically grave and mortal by reason of their matter. That is, there exist acts which, per se and in themselves, independently of circumstances, are always seriously wrong by reason of their object. These acts, if carried out with sufficient awareness and freedom, are always gravely sinful.96
This doctrine, based on the Decalogue and on the preaching of the Old Testament, and assimilated into the kerygma of the apostles and belonging to the earliest teaching of the church, and constantly reaffirmed by her to this day, is exactly verified in the experience of the men and women of all times. Man knows well by experience that along the road of faith and justice which leads to the knowledge and love of God in this life and toward perfect union with him in eternity, he can cease to go forward or can go astray without abandoning the way of God; and in this case there occurs venial sin. This however must never be underestimated, as though it were automatically something that can be ignored or regarded as “a sin of little importance.”
For man also knows, through painful experience, that by a conscious and free act of his will he can change course and go in a direction opposed to God’s will, separating himself from God (aversio a Deo), rejecting loving communion with him, detaching himself from the life principle which God is and consequently choosing death.
95 Cf Council of Trent, Session VI, De Iustificatione, Chap. 2 and Canons 23, 25, 27: Conciliorum Œcumenicorum Decreta, Bologna 1973, 671 and 680f (Denzinger, Heinrich, and Adolf Schönmetzer, Enchiridion symbolorum definitionum et declarationum de rebus fidei et morum [DS] 1573, 1575, 1577).
96 Cf Council of Trent, Session IV De Iustificatione, Chapt. 15: Conciliorum Œcumenicorum Decreta, ed. dt. 677 (DS 1544).
— Reconciliatio et Pænitentia 17