Objection 1. It would seem that not all sins are taken away by Penance. For the Apostle says (Hebrews 12:17) that Esau "found no place of repentance, although with tears he had sought it," which a gloss explains as meaning that "he found no place of pardon and blessing through Penance": and it is related (2 Maccabees 9:13) of Antiochus, that "this wicked man prayed to the Lord, of Whom he was not to obtain mercy." Therefore it does not seem that all sins are taken away by Penance.
Objection 2. Further, Augustine says (De Serm. Dom. in Monte i) that "so great is the stain of that sin (namely, when a man, after coming to the knowledge of God through the grace of Christ, resists fraternal charity, and by the brands of envy combats grace itself) that he is unable to humble himself in prayer, although he is forced by his wicked conscience to acknowledge and confess his sin." Therefore not every sin can be taken away by Penance.
Objection 3. Further, our Lord said (Matthew 12:32): "He that shall speak against the Holy Ghost, it shall not be forgiven him, neither in this world nor in the world to come." Therefore not every sin can be pardoned through Penance.
On the contrary, It is written (Ezekiel 18:22): "I will not remember" any more "all his iniquities that he hath done."
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 Wisdom 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 Proverbs 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 John 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.
Reply to Objection 1. Esau did not truly repent. This is evident from his saying (Genesis 27:41): "The days will come of the mourning of my father, and I will kill my brother Jacob." Likewise neither did Antiochus repent truly; since he grieved for his past sin, not because he had offended God thereby, but on account of the sickness which he suffered in his body.
Reply to Objection 2. These words of Augustine should be understood thus: "So great is the stain of that sin, that man is unable to humble himself in prayer," i.e. it is not easy for him to do so; in which sense we say that a man cannot be healed, when it is difficult to heal him. Yet this is possible by the power of God's grace, which sometimes turns men even "into the depths of the sea" (Psalm 67:23).
Reply to Objection 3. The word or blasphemy spoken against the Holy Ghost is final impenitence, as Augustine states (De Verb. Dom. xi), which is altogether unpardonable, because after this life is ended, there is no pardon of sins. or, if by the blasphemy against the Holy Ghost, we understand sin committed through certain malice, this means either that the blasphemy itself against the Holy Ghost is unpardonable, i.e. not easily pardonable, or that such a sin does not contain in itself any motive for pardon, or that for such a sin a man is punished both in this and in the next world, as we explained in the II-II, 14, 3.
Source: Summa Theologica (New Advent)