A mortal sin, 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. 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 Ghost and the sins that cry to Heaven for vengeance are considered especially serious. This type of sin is 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.