God forgives all sins – so what about mortal sins?

“Amen, I say to you, all sins and all blasphemies that people utter will be forgiven them” (Mk 3:28). These are powerful words. Jesus promises forgiveness without naming any limit to the gravity of guilt. Yet in the very next sentence He warns: “But whoever blasphemes against the Holy Spirit will never have forgiveness, but is guilty of an everlasting sin” (Mk 3:29).

How can this be?

The Catholic Church also teaches that a single mortal sin is enough to lose sanctifying grace. What kind of sin is it that separates us from God? And why should the very forgiveness that Christ offers to all sinners be capable of failing because of human choice?

Mortal Sin and Its Consequences

What Is a Mortal Sin?

A mortal sin is a grave offense against God's commandments, committed with full knowledge and of one's own free will. It destroys supernatural charity toward God and leads to the loss of sanctifying grace.

This idea did not first arise in the Middle Ages. Already the apostle John distinguishes between sin that leads to death and sin that does not lead to death (1 John 5:16–17).

This distinction is rooted in the reality of sin itself. A thoughtless word in an argument between spouses can wound. But it is something else entirely when one of them carries on an affair for months, conceals the unfaithfulness, and keeps inventing new lies to cover it up. The gravity of the guilt here already lies in what is consciously being destroyed.

The Church calls sins that wound love for God without extinguishing it venial sins. The word sounds almost harmless. Yet even smaller offenses leave their mark. An excuse on the phone costs little effort to overcome at first. The next time it comes more easily. Eventually, one lies without giving it much thought. The conscience grows accustomed to things it once would have resisted vehemently.

Mortal sins go much further. With them, a person consciously turns away from God's will in a serious matter. He places his own will above a commandment whose meaning he fully knows. In doing so, he loses the sanctifying grace through which he was united with God.

When Does a Sin Become a Mortal Sin?

A grave act, however, is not in every case equivalent to a mortal sin. In addition to the gravity of the act, full knowledge and free consent on the part of the one acting must also be present.

This can be illustrated (unfortunately) by adultery. A married man begins a relationship with another woman. He knows his obligation to marital fidelity and nevertheless chooses this path. He is acting in a serious matter, knows it is wrong, and wants it anyway, out of free consent. If the man is also aware that he is thereby violating the sixth commandment, full knowledge is present. And with that, the conditions for a mortal sin are more than fulfilled.

Grave offenses also include murder, serious theft, and deliberate blasphemy. The Sunday obligation, too, is not some optional offer from the Church. A Catholic who deliberately misses Holy Mass on Sunday without a serious reason commits an objectively grave sin. On the other hand, someone who is sick or must care for a small child, and therefore cannot attend Mass, does not disregard the Sunday obligation.

When judging a sin, one must therefore distinguish between the act itself and personal responsibility. Severe fear, external compulsion, or a significant impairment of freedom of choice can diminish guilt. Of course, this does not automatically make a wrong act good. But God judges a person according to what he actually recognized and willed.

Yet ignorance does not always excuse. Thomas Aquinas explains in his Summa Theologiae (I–II, q. 76) that a person can also be responsible for his own ignorance. Think of someone who has doubts about whether his behavior is compatible with God's commandments. He could ask a priest or study the teaching of the Church. But he deliberately refrains from doing so, because he fears he might have to change his behavior. In this case, he has chosen on his own to remain ignorant. He cannot later simply appeal to this ignorance.

Why Is It Called a Mortal Sin?

A person who commits a mortal sin does not die on the spot. The next morning they get up, have breakfast, and begin their day. They might even sit in a church pew on Sunday. No one can tell by looking at them what has happened to their soul.

In Baptism, God gave man a share in His own life. Theology calls this gift sanctifying grace. Through it we become children of God and receive communion with Him, which we could never attain by our own strength. “The love of God has been poured into our hearts through the Holy Spirit who has been given to us” (Romans 5:5). This is how Paul describes it.

It is precisely this supernatural love that is destroyed by mortal sin. One loses sanctifying grace and thus the divine life of one's soul. Baptism, however, remains valid, for no sin can remove its indelible mark. Even faith does not necessarily have to be lost.

In its sixth session, the Council of Trent teaches that justification is lost through every mortal sin, even if faith remains (Chapter 15; DH 1544). This means that a Catholic can fully acknowledge the doctrine of the faith and still live in a state of mortal sin. He believes in God, but he consciously does not live in His grace.

Christ Himself warns against a piety that is limited to outward actions: “This people honors me with their lips, but their heart is far from me” (Matthew 15:8). Religious knowledge and pious habits are valuable. But they cannot replace communion with God. The Apostle James goes even further: “You believe that God is one; you do well. Even the demons believe—and shudder” (James 2:19). So even the demons know that God exists. Yet their knowledge does not lead them to Him, for they refuse Him their love and obedience. Merely knowing that God exists is far from believing in Him as Christ demands of us.

Saint Paul names the consequence of sin directly: “For the wages of sin is death” (Romans 6:23). With a mortal sin, of course, no physical death occurs. Whoever commits a mortal sin loses sanctifying grace and thus the supernatural life that God gave them. Communion with God is broken. The danger is that this loss is not necessarily felt. Everyday life goes on. One goes to work, meets friends, and feels joy. One might even continue to consider oneself a good Christian. But the soul remains deprived of sanctifying grace as long as no conversion takes place. If one dies in this state, without repenting of one's sins and accepting God's merciful love, one remains, according to the teaching of the Church, forever separated from Him.

Forgiveness, Conversion, and Eternal Salvation

Can a mortal sinner also be saved?

The history of Sacred Scripture knows sinners whose deeds could hardly be more grave. One of them was none other than King David.

David fell in love with Bathsheba, even though she was married to his soldier Uriah. He had her brought to his palace and slept with her. When Bathsheba became pregnant, David tried to cover up the adultery. He had Uriah recalled from the war, hoping he would spend the night with his wife and later believe the child to be his own. But Uriah refused to go home while his comrades were fighting on the battlefield. Even when David got him drunk, he held to his resolve. Finally, the king resorted to a cruel measure: he ordered his commander to place Uriah at the front line of the fighting and to withdraw the other soldiers so that he would be killed. An act of adultery had become a murder (2 Samuel 11).

David had abused his royal power to conceal his adultery, even bringing about the death of an innocent soldier to do so. At first, he seemed to have gotten away with it. But then God sent the prophet Nathan to him.

Nathan told the king about a rich man who owned a great many sheep. His poor neighbor, on the other hand, had only a single little lamb, which he loved like his own child. When the rich man received a visitor one day, he did not want to slaughter any of his own animals. Instead, he took the poor man's only lamb from him.

David was outraged at this injustice (2 Samuel 12:5). The man deserved to die, he declared angrily: “As the LORD lives, the man who has done this deserves to die” (2 Samuel 12:5). Then Nathan spoke the words David would surely never forget: “You are the man!” (2 Samuel 12:7).

All at once David saw his own guilt before his eyes. He had passed judgment on another, innocent man, while overlooking his own crime. Now he no longer tried to make excuses. He confessed: “I have sinned against the LORD.” And Nathan answered: “The LORD also has put away your sin; you shall not die” (2 Samuel 12:13).

How deeply David's repentance weighed can be glimpsed in Psalm 51, whose very superscription links it to this event. There he prays: “Create in me a clean heart, O God, and put a new and right spirit within me!” (Psalm 51:10). David could not undo his deeds. Even the painful consequences of his guilt remained. But he was allowed to experience that God's mercy was greater than his sin.