From Pope St. John Paul II’s Apostolic Exhortation Reconciliatio et Pænitentia (1984):
But when we ponder the problem of a rebellious will meeting the infinitely just God, we cannot but experience feelings of salutary “fear and trembling,” as St. Paul suggests.92 Moreover, Jesus’ warning about the sin “that will not be forgiven” confirms the existence of sins which can bring down on the sinner the punishment of “eternal death.”
In the light of these and other passages of sacred Scripture, doctors and theologians, spiritual teachers and pastors have divided sins into mortal and venial. St. Augustine, among others, speaks of letalia or mortifera crimina, contrasting them with venialia, levia or quotidiana.93 The meaning which he gives to these adjectives was to influence the successive magisterium of the church. After him, it was St. Thomas who was to formulate in the clearest possible terms the doctrine which became a constant in the church.
In defining and distinguishing between mortal and venial sins, St. Thomas and the theology of sin that has its source in him could not be unaware of the biblical reference and therefore of the concept of spiritual death. According to St. Thomas, in order to live spiritually man must remain in communion with the supreme principle of life, which is God, since God is the ultimate end of man’s being and acting. Now sin is a disorder perpetrated by the human being against this life-principle. “And when through sin, the soul commits a disorder that reaches the point of turning away form its ultimate end, God, to which it is bound by charity, then the sin is mortal; on the other hand, whenever the disorder does not reach the point of a turning away from God, the sin is venial.”94 For this reason venial sin does not deprive the sinner of sanctifying grace, friendship with God, charity and therefore eternal happiness, whereas just such a deprivation is precisely the consequence of mortal sin.
92 Cf Phil 2:12.
93 Cf St. Augustine, De Spintu et Littera, XXVIII: Corpus Scriptorum Ecclesiasticorum Latinorum 60, 202f; Enarrat. in ps. 39, 22: Corpus Christianorum, Latin series [CCL] 38, 441; Enchiridion ad Laurentium de Fide et Spe et Cantate, XIX, 71: CCL 46, 88; In Ioannis Evangelium Tractatus, 12, 3,14: CCL 36, 129.
94 St. Thomas Aquinas, Summa Theologiæ, I-II, q. 72, a. 5.
— Reconciliatio et Pænitentia 17