1. Ethical theory ignores the difference between mortal and venial sin. Although philosophers are aware that there are degrees of seriousness in moral evil, immoral acts considered apart from faith seem to comprise a continuum, from the slight to the very serious. Ethics does not observe the sharp break marked by theology: the distinction between mortal and venial sin.
2. While there are degrees of seriousness in each category, the worst venial sin differs altogether in its significance from the least grave mortal sin (see S.t., 1–2, q. 88, a. 1). As we shall see in the present question, faith requires that this sharp division be acknowledged. Mortal sin will be formally defined in the next question, and the problem of gravity of matter will be more fully explored in chapter sixteen.
3. In the Old Testament, an expiatory offering was required for sins of human frailty and inadvertence (see Lv 4–5). However, other sins, having the character of crimes against the covenant community and its God, could not be expiated. They were punished by death or by cutting the sinner off from the community (see Lv 7.25; 17.8–10, 14; 19.7–8; 20.3; and so on). The words “venial” and “mortal” were not used, but some such distinction is obviously implicit in the difference between faults which could be expiated (“venial” means “pardonable”) and crimes which could not.
4. The New Testament maintains this distinction. In teaching his disciples to pray, Jesus directs them to seek forgiveness for their daily transgressions (see Mt 6.12; Lk 11.4). By contrast, he threatens his determined opponents with the condemnation of Gehenna (see Mt 23.33). Certain sins exclude one from the kingdom forever (see Mt 25.43–46); some are unforgivable in a way that others are not (see Mt 12.31–32; Mk 3.28–30).
5. Similarly, the Epistles mention daily sins of which everyone can be guilty (see Jas 3.2; 1 Jn 1.8). By contrast, there is the slavery to sin which leads to death (see Rom 6.16). Certain sins call for excommunication (see 1 Cor 5.13). The grave sins exclude from the kingdom (see 1 Cor 6.9–10; Gal 5.19–21).
A long tradition has taken a passage in St. Paul as marking the distinction between venial and mortal sin. Paul says people build differently on the foundation which is Jesus, some with gold, silver, and jewels, others with wood, hay, or straw. Judgment will test the quality of each one’s work; one whose building burns because of its poor material can be saved, but as fleeing through a fire. But others utterly destroy God’s temple, for they separate themselves from Jesus; at judgment, these will be destroyed, not saved (see 1 Cor 3.10–17).1
Among the Fathers of the Church, the distinction between venial and mortal sins is clearly marked.2 St. Jerome, for example, says: “There are venial sins and there are mortal sins. It is one thing to owe ten thousand talents, another to owe but a farthing. We shall have to give an accounting for an idle word no less than for adultery. But to be made to blush and to be tortured are not the same thing; not the same thing to grow red in the face and to be in agony for a long time” (FEF 1382). Augustine points out that it is a mistake to make light of lesser sins; they can lead to grave sin. Hence, lesser sins should be confessed and overcome with works of mercy (see FEF 1846). The lesser sins, for which everyone needs pardon, are distinguished from crimes; every crime is a sin, but not every sin is a crime (see FEF 1918). St. Caesar of Arles, writing before the mid-sixth century, briefly lists mortal and venial sins, basing himself on the lists in St. Paul and on the sense of the faithful. Those dominated by mortal sins must do penance, give alms, and amend their lives. The lesser sins can be remitted, even if one dies with them, through purgatorial fire; Casear identifies this with the fire which according to St. Paul will burn away the wood, hay, and straw of those who nevertheless build on Jesus (see FEF 2233).
6. The Church’s teaching, rooted in Scripture, insists on this distinction. Against the Pelagians, the Church teaches that even the upright Christian sins (see DS 228–30/106–8). The Council of Trent teaches that not all sins take away grace; some are venial (see DS 1537/804). Without a special divine privilege, like that given Mary, not even a justified person can altogether avoid venial sin (see DS 1573/833). St. Pius V condemns the severe view that every sin of its nature deserves hell (see DS 1920/1020). Thus the Church insists firmly on the reality of the category called “venial sin.”
7. No less firmly does the Church insist upon the reality of the category called “mortal sin.” The Council of Trent teaches that one can lose the grace of justification not only by sins directly against faith, which cause faith to be lost, but also by a variety of sins which, if unrepented, exclude even believing Christians from the kingdom. Sins other than infidelity also can be grave and enormous; Trent invokes St. Paul (see 1 Cor 6.9–10) in support of this teaching (see DS 1544/808, 1577/837).
8. Trent also insists on the distinction between mortal and venial sin in its teaching on the sacrament of penance. Catholics must confess all mortal sins they can remember after a careful examination of conscience. These can include sins of thought without any external act. Mortal sins have the character of crimes. They must be submitted to the Church, in the person of the confessor, for judgment. Venial sins also may be confessed but they do not have to be (see DS 1679–81/899).
Already in the time of St. Thomas, centuries before Trent, there was substantial agreement among the Church’s teachers not only on the points concerning which Trent insists but also on the sorts of acts which constitute mortal sins. In the centuries since Trent, moral theologians whose works were authorized for use in the formation of confessors reached even more detailed and precise agreement concerning the kinds of acts which are grave matter. This common body of Catholic moral teaching seems to meet the conditions for teaching infallibly proposed by the ordinary magisterium (see 35‑E). Therefore, it not only is a matter of faith that there is a definite line to be drawn between mortal and venial sins; the faithful Catholic in many cases is in a position to say whether a certain kind of act falls on one side of the line or on the other.3
1. See St. Thomas Aquinas, Summa theologiae, 1–2, q. 89, a. 2; St. Augustine, Expositions on the Psalms, 81 (80), 19–20. The fire by which the wood, hay, and straw are consumed is identified by the tradition with the fire of purgatory. See Thomas Deman, O.P., “Péché Mortel et Péché Veniel,” Dictionnaire de Théologie Catholique, 12:225–26.
2. See Hubert Louis Motry, The Concept of Sin in Early Christianity (Washington, D.C.: Catholic University of America, 1920), for a tracing of the concept up to and including Tertullian; summary, 157–58.
3. See Ronald Lawler, O.F.M.Cap., “The Love of God and Mortal Sin,” in Principles of Catholic Moral Life, ed. William E. May (Chicago: Franciscan Herald Press, 1980), 193–219.