What Makes a Sin "Mortal"? Understanding the Church's Teaching

Scripture itself draws a distinction between two categories of wrongdoing, referring to "sin which is mortal" and "sin which is not mortal" (1 John $5:16-17$). The latter is what the Church calls venial or pardonable sin, which wounds our relationship with God but doesn't sever it. To understand what tips the scales and makes a sin mortal—or deadly—we must look to three essential criteria, as reiterated by St. John Paul II: the sin must be of grave matter, committed with full knowledge, and done with deliberate consent. These conditions are critical because they speak to the degree of harm to our soul and the deliberate nature of the choice. For instance, some acts, like murder, are considered intrinsically grave and mortal by their very nature. If we know an action is seriously wrong and freely choose it, we’ve effectively chosen that sin over God, a choice that radically changes our fundamental orientation away from Him.

A common misconception that St. John Paul II explicitly warned against is reducing mortal sin to merely an explicit act of "contempt for God," as if one must internally say, "Away with God!" for the sin to be deadly. On the contrary, mortal sin exists when a person knowingly and willingly chooses something gravely disordered for any reason. Such a choice is already an implicit rejection of God's love and a profound turning away from Him. While factors like ignorance, fear, or duress can certainly diminish or even nullify our responsibility, we must also avoid prejudging in the opposite direction, as if any set of social or psychological factors eliminates the possibility of free will and agency. The Church affirms that we remain moral agents, and if we choose a gravely disordered act with sufficient awareness and freedom, we have chosen sin over God.

So, where does this leave the faithful? Quite simply, it underscores the importance of the Sacrament of Confession. We are obliged to confess all grave sins committed after Baptism, in both kind and number, of which we have knowledge after a diligent examination of conscience. Even if you are unsure whether a sin is mortal, or if you have only venial sins to confess, the Church encourages you to approach the sacrament. Confession is not merely for the removal of grave sin; it is a powerful source of grace, and frequent reception helps prevent "deliberate and unrepented venial sin" from disposing us little by little to commit mortal sin. The sacrament is a cause for great celebration in Heaven over a sinner who repents (Luke $15:7$), and priests are reminded to encourage the faithful toward frequent, devotional confession, for it is a profound benefit to our spiritual life.