Father Pegues continues his discussion of St. Thomas Aquinas’ teaching on virtues and vices by distinguishing between mortal and venial sins. He writes: “A sin is more or less grave according as it is opposed to the degree of good which should be sought by man, and according as the sin is in a greater or a lesser degree voluntary.”
Every sin deserves to be punished “because every sin, as such, is a trespassing of the free will upon ground to which it has no right of entry; and punishment is as it were a restitution, made by the will, of this violation of right.”
Sin revolts against order. Three principles which rule this order are “the divine law always; human authority in those things dependent upon it; and the reason of the sinner according to the degree of his responsibility in sinning.” These principles punish sin: “The reason of the sinner can punish sin in two ways: by remorse and by self-inflicted punishment. Human authority punishes sin by chastisement. The divine law punishes sin in two ways: mediately and immediately.” It inflicts punishment mediately “through the medium of the reason of the sinner and of human authority.”
“Mortal sins are those which kill the soul, in that, by them, charity is lost, which is the principle of the supernatural life.” These sins cause the soul to lose the life which God alone can give. They “render the sinner incapable of making reparation for his sin; and thus, since the sin remains always, the punishment must likewise remain always.”
Sins that are not mortal are venial. A venial sin is “a sin less grave which does not take away the principle of the supernatural life, which is charity or grace.” Thus, reparation can be made for a venial sin “under the ordinary action of grace by a contrary movement of the sinner himself.” Punishment for venial sin is temporal, not eternal. Accordingly, it is called venial, that is, pardonable, from the Latin venia, meaning “pardon.”
The fact that some sins are mortal and others are venial “arises from the nature of the disorder brought about by different sins, and from the greater or lesser voluntariness of sin. . . . There are sins which, of themselves, are directly opposed to the supernatural love of God, which is the principle of the life of the soul, or that they are incompatible with this love; whilst other sins bring about a lesser disorder of an accidental nature which is compatible with the supernatural love of God existing habitually in the soul.” Sins opposed to, or incompatible with, the supernatural love of God are those which “reject the supernatural love of God,” or which imply “an evil and a disorder that disturb the very essence of man’s relation to God, or the mutual relation of men, or the relation of man to himself.” These sins are: spurning the love of God, violating His honor, theft, homicide, adultery, and sins against nature.
Quotations from Thomas Pegues, Catechism of the “Summa Theologica” of Saint Thomas Aquinas, trans. Aelred Whitacre (New York: Benziger, 1922).