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. 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. The sins against the Holy Ghost and the sins that cry to Heaven for vengeance are considered especially serious. This type of sin should be distinguished from a venial sin that 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 Catholic teaching was formalized at the 16th century Council of Trent.
According to Catholic teaching, perfect contrition (or imperfect contrition in the Sacrament of Penance), coupled with a firm resolution to sin no more, can restore a person's relationship with God, as well as God's saving grace. However, as God's mercy and forgiveness is not bound by the Sacrament of Penance, under extraordinary circumstances a mortal sin can be remitted through perfect contrition, which is a human act that arises from a person's love of God. When perfect contrition is the means by which one seeks to restore one's relationship with God, there must also be a resolution to confess all mortal sins (that have not been confessed and absolved previously) in the Sacrament of Penance. A resolution to confess these sins should be made with an act of perfect contrition, regardless of whether or not a person believes that they will have access to the Sacrament of Penance.
The term "mortal sin" is thought to be derived from the New Testament of the Bible. Specifically, it has been suggested that the term comes from the 1 John 5:16–17. In this particular verse, the author of the Epistle writes "There is a sin that leads to death."