A sin is considered to be “mortal” when its quality is such that it leads to a separation of that person from God’s saving grace. Deprived of that grace by their own free will and the free will choice to not repent of it ultimately in sacramental confession, a person places themselves outside of God’s salvation, since God is all grace and cannot stand any imperfection of that grace in His presence. In literal fact, the person choosing to deny themselves God’s saving grace becomes unrecognizable to God and therefore cast into perdition outside of God’s grace.
“Everyone knows—or almost everyone does—that there are morally good actions and morally evil actions. But when is an action not only wrong, but sinful? And particularly mortally sinful? After all, as St. John says, “all wrongdoing is sin, but there is sin which is not mortal” just as “there is sin which is mortal” (1 John 5:16,17). A mortal sin is one that “destroys in us the charity without which eternal beatitude is impossible. Unrepented, it brings eternal death” (CCC 1874). That’s what makes it “mortal,” or deadly: it cuts us off from God forever, unless it is “redeemed by repentance and God’s forgiveness” (CCC 1861).
Thanks be to God, not every evil action is mortally sinful. So how do we know which is which? Just as there are three ingredients in evaluating a moral action (the object, intention, and circumstances), so there are three ingredients in a mortal sin: (1) “grave matter,” (2) “full knowledge,” and (3) “deliberate consent.” And the Catechism is clear that all “three conditions must together be met” (1857).
If all three are met, it’s mortally sinful. Otherwise, “one commits venial sin when, in a less serious matter, he does not observe the standard prescribed by the moral law, or when he disobeys the moral law in a grave matter, but without full knowledge or without complete consent” (CCC 1862). That’s still a problem, since venial sin “weakens charity” and “impedes the soul’s progress in the exercise of the virtues and the practice of the moral good,” and “deliberate and unrepented venial sin disposes us little by little to commit mortal sin,” but venial sin does not (of itself) “deprive the sinner of sanctifying grace, friendship with God, charity, and consequently eternal happiness” (CCC 1863).
What does “grave matter” mean? It means that the sin is serious. But how do we evaluate the seriousness of a sin? The Catechism is explicit about the grave nature of particular sins, including sacrilege (2120), blasphemy (2148), perjury (2152), deliberately avoiding Mass on Sundays and holy days of obligation (2181), murder (2268), hatred of neighbor “when one deliberately desires him grave harm” (2303), prostitution (2355), sexual relations outside marriage (2390), and adulation that “makes one an accomplice in another’s vices or grave sins” (2480). St. Paul likewise gives lists in Galatians 5:19-21 and 1 Corinthians 6:9-10 of sins whose practitioners will not “inherit the kingdom of God.” But none of these lists is exhaustive.
More broadly, the Catechism says that “grave matter is specified by the Ten Commandments” (1858), pointing to Jesus’ words to the rich young man. When the man asks him, “Good Teacher, what must I do to inherit eternal life?”, Jesus answers, “You know the commandments: ‘Do not kill, do not commit adultery, do not steal, do not bear false witness, do not defraud, honor your father and mother’” (Mark 10:17,19). Jesus’ clear implication seems to be that those who do commit adultery, steal, etc. shall not inherit eternal life.
Even here, there are two important caveats. On the one hand, not every violation of the Ten Commandments is a mortal sin. For instance, the person who steals a dollar is not necessarily damned. On the other hand, not every mortal sin is a direct and obvious violation of the Ten Commandments. St. Thomas Aquinas considers this objection directly in considering the sin of gluttony: “Every mortal sin is contrary to a precept of the Decalogue: and this, apparently, does not apply to gluttony.” Aquinas argues that gluttony is an indirect violation of the Third Commandment (keeping the Sabbath holy) by turning us away from holiness. That’s a strange answer, but he explains: “Mortal sins are not all directly opposed to the precepts of the Decalogue [Ten Commandments], but only those which contain injustice: because the precepts of the Decalogue pertain specially to justice and its parts.” In other words, the Ten Commandments lay out “the first principles” of the moral law; they’re not an exhaustive list of every serious sin. The right question is this: is this the kind of behavior that places something else above God or turns me away from God? If so, it’s grave matter.
In considering whether or not a sin is mortal, circumstances matter. For instance, the Catechism points out that “one must also take into account who is wronged: violence against parents is in itself graver than violence against a stranger” (1858). Likewise, stealing from the excess wealth of a millionaire is less evil than stealing the food of a starving man. Taking the example of lying, the Catechism explains how to determine the gravity of a sin: “The gravity of a lie is measured against the nature of the truth it deforms, the circumstances, the intentions of the one who lies, and the harm suffered by its victims. If a lie in itself only constitutes a venial sin, it becomes mortal when it does grave injury to the virtues of justice and charity” (2484).”
Love & His mercy,