A mortal sin (Latin: peccatum mortale), in Catholic theology, is a gravely sinful act which can lead to damnation if a person does not repent of the sin before death. It is alternatively called deadly, grave, and serious. 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. Three conditions must together be met for a sin to be mortal: "Mortal sin is sin whose object is grave matter and which is also committed with full knowledge and deliberate consent." The sin against the Holy Spirit and the sins that cry to Heaven for vengeance are considered especially serious. This type of sin is distinguished from a venial sin in that the latter simply leads to a weakening of a person's relationship with God. Despite its gravity, a person can repent of having committed a mortal sin. Such repentance is the primary requisite for forgiveness and absolution. Teaching on absolution from serious sins has varied somewhat throughout history. The current teaching for Catholics was formalized at the 16th-century Council of Trent.
According to the Church's teaching, mortal sin, which is opposed to God, does not consist only in formal and direct resistance to the commandment of charity. It is equally to be found in this opposition to authentic love which is included in every deliberate transgression, in serious matter, of each of the moral laws. A person therefore sins mortally not only when his action comes from direct contempt for love of God and neighbor, but also when he consciously and freely, for whatever reason, chooses something which is seriously disordered. (Sacred Congregation for the Doctrine of the Faith: Persona Humana, 1975).