Thomas Aquinas

“ In this life the free-will does indeed ever remain subject to change: yet sometimes it rejects that whereby, so far as it is concerned, it can be turned to good. Hence considered in itself this sin is unpardonable, although God can pardon it. ”

“ Nevertheless it is possible for a man, in his first sinful act, to sin against the Holy Ghost by contempt, both on account of his free-will, and on account of the many previous dispositions, or again, through being vehemently moved to evil, while but feebly attached to good. ”

“ First, as regards the punishment, since he that sins through ignorance or weakness, deserves less punishment, whereas he that sins through certain malice, can offer no excuse in alleviation of his punishment. Likewise those who blasphemed against the Son of Man before His Godhead was revealed, could have some excuse, on account of the weakness of the flesh which they perceived in Him, and hence, they deserved less punishment; whereas those who blasphemed against His very Godhead, by ascribing to the devil the works of the Holy Ghost, had no excuse in diminution of their punishment. ”

“ God's gifts whereby we are withdrawn from sin, are two: one is the acknowledgment of the truth, against which there is the "resistance of the known truth," when, namely, a man resists the truth which he has acknowledged, in order to sin more freely: while the other is the assistance of inward grace, against which there is "envy of a brother's spiritual good," when, namely, a man is envious not only of his brother's person, but also of the increase of Divine grace in the world. ”

“ For, by consideration of the Divine judgment, wherein justice is accompanied with mercy, man is hindered from sinning through choice, both by hope, arising from the consideration of the mercy that pardons sins and rewards good deeds, which hope is removed by "despair"; and by fear, arising from the consideration of the Divine justice that punishes sins, which fear is removed by "presumption," when, namely, a man presumes that he can obtain glory without merits, or pardon without repentance. ”

“ Movement both in good and in evil is made, for the most part, from imperfect to perfect, according as man progresses in good or evil: and yet in both cases, one man can begin from a greater (good or evil) than another man does. Consequently, that from which a man begins can be perfect in good or evil according to its genus, although it may be imperfect as regards the series of good or evil actions whereby a man progresses in good or evil.
Reply to Objection 2: This argument considers the sin which is committed through certain malice, when it proceeds from the inclination of a habit. ”

“ I answer that, The above species are fittingly assigned to the sin against the Holy Ghost taken in the third sense, because they are distinguished in respect of the removal of contempt of those things whereby a man can be prevented from sinning through choice. ”

“ Secondly, this may be understood to refer to the guilt: thus a disease is said to be incurable in respect of the nature of the disease, which removes whatever might be a means of cure, as when it takes away the power of nature, or causes loathing for food and medicine, although God is able to cure such a disease. ”