I assure you, however, that if you wish to emend your life in the time that you have, God is so good and merciful that he will grant mercy. He will receive you benevolently in his arms, he will make you participant of the blood of the Lamb shed with such a fire of love, for there is no sinner that is so great as to not obtain mercy. The mercy of God is greater than our evil, whenever we might want to emend ourselves and vomit the corruption of sin through confession, with the intention of preferring death rather than returning to what we had vomited. […] Know that if you do not emend, you will go to the darkest prison imaginable and when what should occur through confession and the rejection sin does not, there is no need for anyone to put the debtor into prison, rather, he himself will go to hell in the company of the demons. (Saint Catherine of Siena. Letter 21 – To someone whose name has been withheld reproving grave mortal sins)

Having then all this in your mind, show forth a life worthy of the love of Him who calls you, and of your citizenship in that world, and of the honor that is given you. […] Now then, having to partake of such blessings […] Or rather, what punishment will you not have to suffer, who after so great a gift art running to your former vomit? For no longer are you punished merely as a man, but as a son of God that has sinned; and the greatness of your honor becomes a mean of bringing a sorer punishment on you. Since we too punish not equally slaves that do wrong, and sons committing the same offense; and most of all when they have received some great kindness from us. For if he who had paradise for his portion, for one disobedience underwent such dreadful things after his honor; we, who have received Heaven, and have become joint heirs with the Only Begotten, what excuse shall we have, for running to the serpent after the dove? For it will be no longer, ‘Dust you are, and unto dust shall you return’ (Gen 3:19), and you ‘till the ground,’ and those former words, that will be said to us; but what is far more grievous than these, the ‘outer darkness’ (Mt 25:30), the bonds that may not be burst, the venomous worm, the ‘gnashing of teeth;’ and this with great reason. For he that is not made better even by so great a benefit, would justly suffer the most extreme, and a yet more grievous punishment. (Saint John Chrysostom. Homilies on Saint Matthew, Homily 12, no 3-4)

‘The Lord is full of compassion and mercy: long-suffering, and of great mercy’ (Ps 102:8). Why so long-suffering? Why so great in mercy? Men sin and live; sins are added on, life continues: men blaspheme daily, and ‘He makes His sun to rise over the good and the wicked’ (Mt 5:45). On all sides He calls to amendment, on all sides He calls to repentance, He calls by the blessings of creation, He calls by giving time for life, He calls through the reader, He calls through the preacher, He calls through the innermost thought by the rod of correction, He calls by the mercy of consolation: ‘He is long-suffering, and of great mercy.’ But take heed lest by ill using the length of God’s mercy, you store up for yourself, as the Apostle says, wrath in the day of wrath….For some there are who prepare to turn, and yet put it off, and in them cries out the raven’s voice, ‘Cras! Cras!’ […] How long, Tomorrow! Tomorrow!? Look to your last morrow: since you know not what is your last morrow, let it suffice that you have lived up to this day a sinner. You have heard, often you are wont to hear, you have heard today also; daily you hear, and daily you amend not.… […] What terrible anxieties here in this life? I omit hell. Beware lest you even now become a hell unto yourself. The whole of this, my brethren, is the result of His anger: and when you have turned yourself unto works of righteousness, you cannot but toil upon earth; and toil ends not before life ends. (Saint Augustine of Hippo. Commentary on the Psalms, Psalm 103, no. 13-14 )

For thou, my God, thou hast made all things by the power, and art wonderful in all thy doings; yet art thou most wonderful, and exceeding glorious in thy Works of pity and love. In this sense too is that most true, which thou speakest of thyself by the moth of thy servants. ‘The Lord is good to all, and his tender mercies are over all his works’ (Ps 145:9). And what was said of one particular person, we may most truly apply to thy people in general, my mercy will I not take from him (Ps 89:24, 28). For thou abhorrest, despises, forsaketh no man; but such only as, lost to all sense of their own duty and happiness, do first despise and forsake thee. Hence it is that thou dost not only not strike when thou art not angry, but even when thou art most justly so. Thou givest good things liberally, upon the request of those wretches who have provoked thee to anger. O my God, the horn of my salvation, and my refuge, I am sadly sensible that I am one of those miserable wretches; I have provoked thy wrath, and done evil in thy sight; and yet thou holdest thy hand. I have sinned, thou hast suffered; I have offended, and still thou bearest with me. If I repent thou sparest; if I return, thou receivest me with open arms; nay, even while I delay, thou waitest patiently for my coming back to thee. Thou callest me to thee, when I go astray, thou invitest me while I am deaf to thy gracious calls; thou stayest till I shake off my wicked sloth; and, when thy prodigal child at last bethinks himself, thou meetest and embracest him most gladly. Thou instructest my ignorance, comfortst my sorrows, keepest me from falling, raiseth me up when I am fallen, givest when I ask, art found when I seek thee, and openst the door when I knock (Mt 7:7). (Saint Augustine of Hippo. Meditations, Book I, Ch. 2, pg. 12-13)

And lest haply he should seem to have given impunity for sins, in that he said, ‘He is faithful and just to cleanse us from all iniquity;’ and men henceforth should say to themselves, ‘Let us sin, let us do securely what we will, Christ purges us, is faithful and just, purges us from all iniquity’: He takes from you an evil security, and puts in an useful fear. To your own hurt you would be secure; you must be solicitous. For ‘He is faithful and just to forgive us our sins,’ provided you always displease yourself, and be changing until you be perfected. […] if from the infirmity of this life sin shall overtake you, see to it straightway, straightway be displeased, straightway condemn it; and when you have condemned, you shall come assured unto the Judge. (Saint Augustine of Hippo. Homilies on the First Epistle of Saint John. Homily 1, no. 7)

Run, my brethren, lest the darkness lay hold of you. […] Awake, then, while it is day: the day shines, Christ is the day. He is ready to forgive sins, but to them that acknowledge them; ready to punish the self-defenders, who boast that they are righteous, and think themselves to be something when they are nothing. (Saint Augustine of Hippo. Tractates on the Gospel of Saint John, Tractate 12, no. 14)

If ‘God be light, and in Him is no darkness at all, and we must have fellowship with Him,’ then from us also must the darkness be driven away, that there may be light created in us, for darkness cannot have fellowship with light. […] You say you have fellowship with God, and you walk in darkness; ‘and God is light, and in Him is no darkness at all:’ then how should there be fellowship between light and darkness? […] But sins are darkness, as the Apostle says of the devil and his angels, that they are ‘rulers of this darkness’ (Eph 6:12). He would not call them of darkness, save as rulers of sins, having lordship over the wicked. Then what are we to do, my brethren? Fellowship with God must be had, other hope of life eternal is none; […] Let us walk in the light, as He is in the light, that we may be able to have fellowship with Him. (Saint Augustine of Hippo. Homilies on the First Epistle of Saint John, Homily 1, no. 5)

Say not: the mercy of the Lord is great, He will have mercy on the multitude of my sins. Why does he tell you not to say that the mercy of God is great? Attend to the words contained in the following verse: for mercy and wrath quickly come from Him, and His wrath looketh upon sinners. The mercy of God is different from the acts of His mercy. The former is infinite, the latter are finite. God is merciful but He is also just. Saint Basil says that sinners also consider God as merciful, and ready to pardon, but not as just and prepared to inflict punishment. Of this the Lord complained one day to Saint Bridget: ‘I am just and merciful. Sinners regard me only as merciful’. Saint Basil’s words are, ‘Sinners only consider God merciful and ready to pardon. But God is just and prepared to inflict punishment.’ God is just, and being just He must punish the ungrateful […] Mercy, as the Divine Mother said, is promised to those who fear, and not to those who insult the Lord. And His mercy to them that fear him. Some rash sinners will say: God has hitherto shown me so many mercies, why should He not hereafter treat me with the same mercy? My answer: He will show you mercy, if you wish to change your life. But if you intend to continue to offend Him, He tells you that he will take vengeance on your sins by casting you into hell. (Saint Alphonsus de Liguori. Sermon 41 – for the Twelfth Sunday after Pentecost: On the abuse of divine mercy – Spanish)

The best thing then is, to avoid sin in the first instance: the next to it, is to feel that we sin, and thoroughly amend ourselves. But if we have not this, how shall we pray to God, and ask forgiveness of our sins, we who take no account of these matters? For when you yourself who hast offended art unwilling to know so much as this very fact, that you have sinned; for what manner of offenses will you entreat God for pardon? For what you know not? And how will you know the greatness of the benefit? Tell therefore your offenses in particular, that you may learn for what you receive forgiveness, that so you may become grateful towards your Benefactor. […] but when the God of all is provoked, we gape, and throw ourselves back, and live in luxury and in drunkenness, and do all things as usual. And when shall we be able to propitiate Him? And how shall we by this very thing fail to provoke Him so much the more? For not so much sinning, as signing without even pain, causes in Him indignation and wrath. (Saint John Chrysostom. Homilies on Saint Matthew, Homily 14, no. 4)

For the wicked are at war with God, who is offended beyond belief at their crimes; hence the Apostle says: Wrath and indignation, tribulation and anguish upon every soul of man that worketh evil. Although the sinful act is transient, yet the sin by its guilt and stain remains; and the imminent wrath of God pursues it, as the shadow does the body. (Catechism of Trent, 4500)

Perhaps one becomes perplexed by this word of the Prophet: ‘He who loves iniquity, hates his soul’ (Ps 10:6). But I add: he also hates his own flesh. Does he not treat it with hate, in fact, when every day he accumulates for it the torments of hell, and he accumulates, due to his hardening in evil and the impenitence of his heart, an abundance of wrath for the day of revenge? It is true that we should judge much less from his intention than by the effects [of his deeds] that the sinner is the enemy of his body, just as of his soul. For example, the dissolute who, while drowsing his reason endeavors to do evil to himself, shows himself to be the enemy of his body. Yet is there worse dissolution than impenitence of the heart and obstinacy in sin? It is not just on his body that the miserable raises a violent hand, but his own soul that he wounds and tears. If you have ever seen a man rub his hands together until they start bleeding, you have a clear example of what the sinner does. (Saint Bernard of Clairvaux. Of Conversion: a sermon to the clergy, Ch. IV, no. 5)

From both, then, men are in danger; both from hoping and despairing, from contrary things, from contrary affections. Who is deceived by hoping? He who says, God is good, God is merciful, let me do what I please, what I like; let me give loose reins to my lusts, let me gratify the desires of my soul. Why this? Because God is merciful, God is good, God is kind. These men are in danger by hope. And those are in danger from despair, who, having fallen into grievous sins, fancying that they can no more be pardoned upon repentance, and believing that they are without doubt doomed to damnation, do say with themselves, We are already destined to be damned, why not do what we please with the disposition of gladiators destined to the sword. This is the reason that desperate men are dangerous: for, having no longer anything to fear, they are to be feared exceedingly. Despair kills these; hope, those. The mind is tossed to and fro between hope and despair. You have to fear lest hope slay you; and, when you hope much from mercy, lest you fall into judgment: again, you have to fear lest despair slay you, and, when you think that the grievous sins which you have committed cannot be forgiven you, you do not repent, and you incur the sentence of Wisdom, which says: ‘I also will laugh at your perdition’ (Prov 1:26). How then does the Lord treat those who are in danger from both these maladies? To those who are in danger from hope, He says, ‘Be not slow to be converted to the Lord, neither put it off from day to day; for suddenly His anger will come, and in the time of vengeance, will utterly destroy you’ (Sir 5:8-9). To those who are in danger from despair, what does He say? ‘In what day soever the wicked man shall be converted, I will forget all his iniquities’ (Ez 18:21). (Saint Augustine of Hippo. Tractates on the Gospel of Saint John, Tractate 33, no. 8)

There exists an unfaithful confidence, which only attracts a curse: and it is that which a man has when he sins with the hope of pardon. But this should not be called confidence, rather, insensibility and pernicious falsity. For, what confidence could exist in one that does not even have the thought of danger? How will he find a remedy against fear when he does not fear? Hope is a consolation; but the one who applauds himself for having worked evil, and finds joy in vile things does not need consolation. […] Let us examine our ways and our affections, let us weigh with scrupulous attention all the dangers that threaten us. May each one declare full of fear: ‘I will go to the gates of hell; so that we no longer breath anything other than the mercy of God. (Saint Bernard of Clairvaux. Sermon III on the Annunciation of the Virgin)

If any man wishes to know the power of the grace of God, let him cast his eyes on the good thief. […] Nevertheless, by the assistance of God’s grace, when the gates of heaven seemed shut against him, the jaws of hell open to receive, and the sinner himself as far removed as possible from life eternal, he was suddenly illuminated from on high, his thoughts were directed into the proper channel, and he confessed Christ to be innocent and the King of the world to come, and, like a minister of God, rebuked his fellow-thief, persuaded him to repent, and commended himself humbly and devoutly to Christ. […] On the other hand, in order to let us see the extent of human weakness, the bad thief is not converted either by the immense charity of Christ, Who so lovingly prayed for His executioners, or by the force of his own sufferings, or by the admonition and example of his companion, or by the unusual darkness, the splitting of rocks, or the conduct of those who, after the death of Christ, returned to the city striking their breasts. […] For if one thief cooperated with the grace of God in that last moment, the other rejected it, and met his final doom. (Saint Robert Bellarmine. The seven words on the Cross, Book 1, ch. VI)

What great things did the thief do, you ask, who from the cross passed directly to paradise? I will demonstrate his virtue in a few words. While below at the foot of the cross Peter denied, he above confessed! And this I do not say accusing Peter, far be it from me, but rather to manifest the virtue of the thief. The disciple did not resist the threats of a young maid without importance, while the thief, on the contrary, contemplating all of the people that surrounded him, crying out and proffering blasphemies and insults, he did not mind this, nor did he think on the present dishonor of the Crucified one; but rather, passing over all this with the eyes of faith, he pays no attention to these vile impediments, he recognizes the King of Heaven; and with his spirit prostrated before Him, he said: ‘Remember me when You come into your kingdom!’ […] There was, in effect, another thief crucified together with him, so that it would be fulfilled that he would be numbered among the criminals […] Then: the other thief insulted him. Do you see the difference between one thief and the other? Both are on the cross! Both due to their dishonest life! Both for their iniquity! But they did not have the same end! For one received as inheritance the Kingdom of Heaven, and the other was cast into hell. […] There was a difference between the two thieves: one insulted, the other adored; one blasphemed, the other blessed and even accused the blasphemer with these words: ‘Have you no fear of God, we have been condemned justly, for the sentence we received corresponds to our crimes!’ […] God is a just judge, and his justice goes forth like light that cannot be darkened by ignorance or by shadows. (Saint John Chrysostom. Homily I, On the Cross and the thief, PG 49, 403-4)

The blessed thief thus taught those that stood by, uttering the words by which he rebuked the other. But when he saw that the ears of those who stood by were stopped up, he turns to Him who knows the hearts; for it follows, And he said to Jesus, Lord, remember me when you come into your kingdom. You behold the Crucified, and you acknowledge Him to be your Lord. You see the form of a condemned criminal, and you proclaim the dignity of a king. Stained with a thousand crimes, you ask the Fountain of righteousness to remember your wickedness, saying, But I discover your hidden kingdom; and you turn away my public iniquities, and accept the faith of a secret intention. Wickedness usurped the disciple of truth, truth did not change the disciple of wickedness. (Saint John Chrysostom quoted by Saint Thomas Aquinas. Catena Aurea on Lk 23:38-43)

It is not the penalty but the cause that makes the martyr. Three were crucified. Identical is the penalty, but diverse the cause. One will be damned, the other saved; in the middle the One who damns and saves. He punishes one, saves the other. That cross was the tribunal. (Saint Augustine. Discourse 328 – In Natali Martyrum, no. 8/6)

And this indeed was the purpose of the merciful Jesus, when He showed His Heart to us bearing about it the symbols of the passion and displaying the flames of love, that from the one we might know the infinite malice of sin, and in the other we might admire the infinite charity of Our Redeemer, and so might have a more vehement hatred of sin, and make a more ardent return of love for His love. (Pius XI. Encyclical Miserentissimus Redemptor, no. 11, May 8, 1928)

And prelates, placed in the position of the prelacy of Christ ton earth, offered Me the sacrifice of justice with holy and upright lives. The pearl of justice, with true humility, and the most ardent love shone in them, and in their subjects, with the light of discretion. […] And, because they first had done justice to those under them, wishing to see them live virtuously, and correcting them without any servile fear, because they were not thinking of themselves, but solely of My honour and the salvation of souls […] They have followed His footsteps, and therefore did they correct them, and did not let their members become putrid for want of correcting, but they charitably corrected them with the unction of benignity, and with the sharpness of fire, cauterizing the wound of sin with reproof and penance, little or much, according to the graveness of the fault. And, in order to correct it and to speak the truth, they did not even fear death. They were true gardeners who, with care and holy tears, took away the thorns of mortal sins, and planted plants odoriferous of virtue. Wherefore, those under them lived in holy, true fear, and grew up like sweet smelling flowers in the mystic body of the holy Church […] without any sin, for they balanced exactly the scales of holy justice […] And this justice was and is that pearl which shines in them, and which gave peace and light in the midst of the people and caused holy fear to be with them, and unity of heats. And I would that thou know that, more darkness and division have come into the world amongst seculars and religious and the clergy and pastors of the holy Church, through the lack of the light of justice and the advent of darkness of injustice, than from any other cause. […] So, were the prelate, or any other lord having subjects, on seeing one putrefying from the corruption of mortal sin, to apply to him the ointment of soft words and encouragement alone, without reproof, he would never cure him, but the putrefaction would rather spread to the other members, who, with, him, form one body under the same pastor. (Saint Catherine of Siena. The Dialogue. Treatise on prayer – Of the excellence, virtues, and holy works of virtuous and holy ministers; and how such are like the sun. pg. 245-247)

Another way to do harm is that of those who speak of religious matters as if they were to be considered according to the norms and convenience of this passing life, forgetting the eternal life to come: they speak brilliantly of the benefits that the Christian religion has bequeathed to humanity, but not of the obligations it demands; they preach the charity of Jesus Christ our Savior, but say nothing of his justice. The fruit that such preaching produces is insignificant, because any worldling who hears it becomes convinced that he is a good Christian, and that he has no need to change his life, as long as he says: I believe in Jesus Christ. What kind of fruits do such preachers expect to reap? They certainly have no intention other than that of gaining at any cost the favor of their listeners, flattering them, and, as long as they see the church full, they do not care if the souls of the faithful remain empty. Consequently, they do not even mention sin, the four last things, or any other important topic. Rather, to obtain acclaim and applause, they use complacent language, with eloquence more fitting for worldly speeches than an apostolic and sacred sermon. Against such preachers, Saint Jerome wrote (Ad Nep.): ‘When you teach in the church, you should not provoke the acclamation of the congregation, but rather, compunction: may the tears of your listeners be your praise.’ (Pius X. Motu proprio Sacrorum antistitum, September 1, 1910)

We feel We owe no greater debt to Our office and to Our time than to testify to the truth with Apostolic firmness: ‘to give testimony to the truth.’ This duty necessarily entails the exposition and confutation of errors and human faults; for these must be made known before it is possible to tend and to heal them. ‘You shall know the truth and the truth shall make you free’ (Jn 8:32). (Pius XII. Encyclical Summi pontificatus, no. 14, October 20, 1939)

Amidst all the aberrations of human thought, infatuated by a false emancipation from every law and curb; and amidst the awful corruptions of human malice, the Church rises up like a bright lighthouse warning by the clearness of its beam every deviation to right or left from the way of truth, and pointing out to one and all the right course that they should follow. Woe if ever this beacon should be, we do not say extinguished, for that is impossible owing to the unfailing promises on which it is founded, but if it should be hindered from shedding far and wide its beneficent light! […] That it has not fallen still lower down the slope of error and vice is due to the guidance of the light of Christian truth that always shines in the world. Now the Church exercises her ‘ministry of the word’ through her priests of every grade of the Hierarchy, in which each has his wisely allotted place. These she sends everywhere as unwearied heralds of the good tidings which alone can save and advance true civilization and culture, or help them to rise again. (Pius XI. Encyclical Ad catholici sacerdotii, no. 24, December 20, 1935)

The Lord said: ‘Peter, do you love me? Lord, you know that I love you. Tend my sheep’ (Jn 21:15-17). How is it possible to confide such dear sheep to a man who himself did not love? You know that what is asked of the administrators is that, at least, they be of a proven fidelity. Woe to disloyal servants, who intend to reconcile others, when they themselves are nor reconciled, as if they could make men just in any case! Woe to these children of wrath, who present themselves as ministers of mercy. They are rebellious, and they do not fear to usurp the esteem and the renown that corresponds to the peaceful. They are false and liars, who present themselves as faithful mediators of peace and are fattened on the sins of the people. They are wretched, slaves of their base desires; their conduct does not please God, and nonetheless they pretend to want to placate him. (Saint Bernard of Clairvaux. Of Conversion: a sermon to the clergy, Ch. XIX, no. 32)

However, on account of the importance of the obligation, God wished to make the law, which commands His own divine and most holy name to be honored, a distinct Commandment, expressed in the clearest and simplest terms. The above observation should strongly convince the pastor that on this point it is not enough to speak in general terms; that the importance of the subject is such as to require it to be dwelt upon at considerable length, and to be explained to the faithful in all its bearings with distinctness, clearness and accuracy. This diligence cannot be deemed superfluous, since there are not wanting those who are so blinded by the darkness of error as not to dread to blaspheme His name, whom the Angels glorify. Men are not deterred by the Commandment laid down from shamelessly and daringly outraging Him divine Majesty every day, or rather every hour and moment of the day Who is ignorant that every assertion is accompanied with an oath and teems with curses and imprecations? To such lengths has this impiety been carried, that there is scarcely anyone who buys, or sells, or transacts business of any sort, without having recourse to swearing, and who, even in matters the most unimportant and trivial, does not profane the most holy name of God thousands of times. It therefore becomes more imperative on the pastor not to neglect, carefully and frequently, to admonish the faithful how grievous and detestable is this crime. (Catechism of Trent, 3200)

As, however, the dread of punishment has often a powerful effect in checking the tendency to sin, the pastor, in order the more effectively to move the minds of men and the more easily to induce to an observance of this Commandment, should diligently explain the remaining words, which are, as it were, its appendix: For the Lord will not hold him guiltless that shall take the name of the Lord his God in vain. […] The pastor should urge and insist on this consideration with greatest earnestness. in order that the faithful may be made sensible of the grievousness of the crime, may detest it still more, and may employ increased care and caution to avoid its commission. He should also observe how prone men are to this sin, since it was not sufficient to give the command, but also necessary to accompany it with threats. […] He should next show that God has appointed no particular punishment. The threat is general; it declares that whoever is guilty of this crime shall not escape unpunished. The various chastisements, therefore, with which we are every day visited, should warn us against this sin. It is easy to conjecture that men are afflicted with heavy calamities because they violate this Commandment; and if these things are called to their attention, it is likely that they will be more careful for the future. Deterred, therefore, by a holy dread, the faithful should use every exertion to avoid this sin. If for every idle word that men shall speak, they shall render an account on the day of judgment, what shall we say of those heinous crimes which involve great contempt of the divine name? (Catechism of Trent, 3200)

A person who in a public show or speech, in published writing, or in other uses of the instruments of social communication utters blasphemy, gravely injures good morals, expresses insults, or excites hatred or contempt against religion or the Church is to be punished with a just penalty. (Code of Canon Law, Can. 1369)

All sins and blasphemies are not indeed remitted to all men, but to those who have gone through a repentance in this life sufficient for their sins. (Saint Bede the Venerable quoted by Saint Thomas Aquinas. Catena Aurea on Mk 3:23-30)

Blasphemy is directly opposed to the second commandment. It consists in uttering against God – inwardly or outwardly – words of hatred, reproach, or defiance; in speaking ill of God; in failing in respect toward him in one’s speech; in misusing God’s name. Saint James condemns those ‘who blaspheme that honorable name [of Jesus] by which you are called.’ The prohibition of blasphemy extends to language against Christ’s Church, the saints, and sacred things. […] the misuse of God’s name to commit a crime can provoke others to repudiate religion. Blasphemy is contrary to the respect due God and his holy name. It is in itself a grave sin. (Catechism of the Catholic Church, no. 2148)

The word blasphemy seems to denote the disparagement of some surpassing goodness, especially that of God. Now God, as Dionysius says (Div. Nom. I), is the very essence of true goodness. Hence whatever befits God, pertains to His goodness, and whatever does not befit Him, is far removed from the perfection of goodness which is His Essence. Consequently whoever either denies anything befitting God, or affirms anything unbefitting Him, disparages the Divine goodness. […] If it is in thought only, it is blasphemy of the heart, whereas if it betrays itself outwardly in speech it is blasphemy is opposed to confession of faith. […] He that speaks against God, with the intention of reviling Him, disparages the Divine goodness, not only in respect of the falsehood in his intellect, but also by reason of the wickedness of his will, whereby he detests and strives to hinder the honor due to God, and this is perfect blasphemy. (Saint Thomas Aquinas. Summa Theologica II-II, q. 13, a.1)

It is written (Lev 24:16): ‘He that blasphemeth the name of the Lord, dying let him die.’ Now the death punishment is not inflicted except for a mortal sin. Therefore blasphemy is a mortal sin. I answer that, As stated above (I-II q. 72, a. 5), a mortal sin is one whereby a man is severed from the first principle of spiritual life, which principle is the charity of God. Therefore whatever things are contrary to charity, are mortal sins in respect of their genus. Now blasphemy, as to its genus, is opposed to Divine charity, because, as stated above (a. 1), it disparages the Divine goodness, which is the object of charity. Consequently blasphemy is a mortal sin, by reason of its genus. (Saint Thomas Aquinas. Summa Theologica, II-II, q. 13, a.2)

On Isaiah 18:2, ‘To a terrible people,’ etc. a gloss says: ‘In comparison with blasphemy, every sin is slight.’ I answer that, As stated above (a. 1), blasphemy is opposed to the confession of faith, so that it contains the gravity of unbelief: while the sin is aggravated if the will’s detestation is added thereto, and yet more, if it breaks out into words, even as love and confession add to the praise of faith. Therefore, since, as stated above (II-II q. 10, a. 33), unbelief is the greatest of sins in respect of its genus, it follows that blasphemy also is a very great sin, through belonging to the same genus as unbelief and being an aggravated form of that sin. (Saint Thomas Aquinas. Summa Theologica II-II, q. 13, a.3)

Now those who are in hell retain their wicked will which is turned away from God’s justice, since they love the things for which they are punished, would wish to use them if they could, and hate the punishments inflicted on them for those same sins. They regret indeed the sins which they have committed, not because they hate them, but because they are punished for them. Accordingly this detestation of the Divine justice is, in them, the interior blasphemy of the heart: and it is credible that after the resurrection they will blaspheme God with the tongue, even as the saints will praise Him with their voices. […] Whoever dies in mortal sin, bears with him a will that detests the Divine justice with regard to a certain thing, and in this respect there can be blasphemy in him. (Saint Thomas Aquinas. Summa Theologica II-II, q. 13, a.4)