information:classification, structural:reformatting, structural:expansion
Role in relation to sin

According to the doctrine of the Catholic Church, those who die in God's grace and friendship imperfectly purified, although they are assured of their eternal salvation, undergo a purification after death, so as to achieve the holiness necessary to enter the joy of God.

Unless "redeemed by repentance and God's forgiveness", mortal sin, whose object is grave matter and is also committed with full knowledge and deliberate consent, "causes exclusion from Christ's kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back." Such sin "makes us incapable of eternal life, the privation of which is called the 'eternal punishment' of sin".

Venial sin, while not depriving the sinner of friendship with God or the eternal happiness of heaven, "weakens charity, manifests a disordered affection for created goods, and impedes the soul's progress in the exercise of the virtues and the practice of the moral good; it merits temporal punishment", for "every sin, even venial, entails an unhealthy attachment to creatures, which must be purified either here on earth, or after death in the state called purgatory. This purification frees one from what is called the 'temporal punishment' of sin".

"These two punishments must not be conceived of as a kind of vengeance inflicted by God from without, but as following from the very nature of sin. A conversion which proceeds from a fervent charity can attain the complete purification of the sinner in such a way that no punishment would remain."

Joseph Ratzinger has paraphrased this as: "Purgatory is not, as Tertullian thought, some kind of supra-worldly concentration camp where man is forced to undergo punishment in a more or less arbitrary fashion. Rather it is the inwardly necessary process of transformation in which a person becomes capable of Christ, capable of God, and thus capable of unity with the whole communion of saints".

This purification from our sinful tendencies has been compared to rehabilitation of someone who needs to be cleansed of any addiction, a gradual and probably painful process. It can be advanced during life by voluntary self-mortification and penance and by deeds of generosity that show love of God rather than of creatures. If not completed before death, it can still be needed for entering the divine presence.

A person seeking purification from sinful tendencies is not alone. Because of the communion of saints: "the holiness of one profits others, well beyond the harm that the sin of one could cause others. Thus recourse to the communion of saints lets the contrite sinner be more promptly and efficaciously purified of the punishments for sin". The Catholic Church states that, through the granting of indulgences for manifestations of devotion, penance and charity by the living, it opens for individuals "the treasury of the merits of Christ and the saints to obtain from the Father of mercies the remission of the temporal punishments due for their sins".