Dietrich von Hildebrand, quoted from Transformation in Christ (Ignatius Press 2001):
Objectively, even, contrition as such involves a radical inward change (and a change that cannot be accomplished without contrition). The painful evocation and condemnation of past sins, the groping for a new basis of orientation, the movement of reconversion to God – these aspects by themselves testify to an essential inward change. But all this is far from being equivalent to an abolition of the guilt incurred. The disuniting effect of the latter persists, and continues to lie in the path of a reconciliation with God. That guilt can only be eliminated by God’s act of pardon, and be compensated for by the blood of Christ, of which it is said in the hymn of St. Thomas: “Of which a single drop, for sinners split, can purge the entire world from all its guilt.”
The sacrament of Penance, strictly speaking, is not indispensable for redeeming man from his guilt. In regard to a venial sin, the act of repentance itself may be an adequate substitute for the sacrament; in regard to a grave sin, an act of perfect contrition may similarly suffice, provided that confession is impracticable – just as in the baptism of desire and the baptism of blood, an inner act and a heroic action, respectively may stand for the sacrament of Baptism. But even in such cases it is not the indwelling force of the human act of penitence as such which abolishes the guilt; this is done always, and solely, by Christ through His death on the Cross. The change of heart, as implied in contrition, merely opens the path for the influx of the redeeming blood of Christ. Penitence reestablishes the link with Christ, by virtue of which the fruits of Christ’s deed of redemption may be applied to us.
Even the penitence of the Prophets, and all of those who lived before Christ, did not achieve the removal of guilt on its own strength: here, too, the forgiveness of guilt was due to the redeeming sacrifice of Christ.