I answer that, The fact that a sin cannot be taken away by Penance may happen in two ways: first, because of the impossibility of repenting of sin; secondly, because of Penance being unable to blot out a sin. In the first way the sins of the demons and of men who are lost, cannot be blotted out by Penance, because their will is confirmed in evil, so that sin cannot displease them as to its guilt, but only as to the punishment which they suffer, by reason of which they have a kind of repentance, which yet is fruitless, according to Wis. 5:3: "Repenting, and groaning for anguish of spirit." Consequently such Penance brings no hope of pardon, but only despair. Nevertheless no sin of a wayfarer can be such as that, because his will is flexible to good and evil. Wherefore to say that in this life there is any sin of which one cannot repent, is erroneous, first, because this would destroy free-will, secondly, because this would be derogatory to the power of grace, whereby the heart of any sinner whatsoever can be moved to repent, according to Prov. 21:1: "The heart of the king is in the hand of the Lord: whithersoever He will He shall turn it."
It is also erroneous to say that any sin cannot be pardoned through true Penance. First, because this is contrary to Divine mercy, of which it is written (Joel 2:13) that God is "gracious and merciful, patient, and rich in mercy, and ready to repent of the evil"; for, in a manner, God would be overcome by man, if man wished a sin to be blotted out, which God were unwilling to blot out. Secondly, because this would be derogatory to the power of Christ's Passion, through which Penance produces its effect, as do the other sacraments, since it is written (1 Jn. 2:2): "He is the propitiation for our sins, and not for ours only, but also for those of the whole world."
Therefore we must say simply that, in this life, every sin can be blotted out by true Penance.