What Is a Mortal Sin?

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). So what sin is mortal and what is not? 

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. 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” for an act to be mortally sinful (1857).  

Short of this, “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). Although venial sin does not “deprive the sinner of sanctifying grace, friendship with God, charity, and consequently eternal happiness,” it “weakens charity” and “impedes the soul’s progress in the exercise of the virtues and the practice of the moral good,” such that “deliberate and unrepented venial sin disposes us little by little to commit mortal sin” (CCC 1863). 

We’ll look at “full knowledge” and “deliberate consent” in the following questions, but what does “grave matter” mean? It means that the sin is serious. But how do we evaluate the seriousness of a sin? Coming back to the threefold distinction of object, intention, and consequences: the “matter” in question is the object of the act. Grave matter means that the objective character of the act performed is seriously disordered by its very nature. 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 lists in Galatians 5:19-21 and 1 Corinthians 6:9-10 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 commit adultery, steal, etc. shall not inherit eternal life. 

Even here, there are a few important caveats. First, the Ten Commandments lay out “the first principles” of the moral law; they’re not an exhaustive list of every serious sin.cli The right question in determining grave matter 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. 

Second, not every violation of the Ten Commandments is a mortal sin. For instance, the person who steals a dollar is not necessarily damned. 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). 

Happily, with all these factors to take into consideration, the Lord does not require us to calculate exact gravity of our sin before turning to him for mercy. He alone probes “the division of soul and spirit, of joints and marrow, discerning the thoughts and intentions of the heart” (Heb. 4:12). He alone is “lawgiver and judge, he who is able to save” (James 4:12). 

Mortal sin, venial sin, and everything in between. There's a lot to unpack when it comes to the Catholic Faith. Get your copy today of the NEW Big Book of Catholic Answers, Vol. 3!