Sin is not just a psychological and social matter, but an event that corrodes the relationship with God, violating his law, refusing his plan in history and overturning his set of values, ‘putting darkness for light and light for darkness’, in other words, ‘calling evil good and good evil’ (Is 5:20). Before finally injuring man, sin is first and foremost a betrayal of God. (John Paul II. General audience, no. 3, May 8, 2002)

The Church, taking her inspiration from Revelation, believes and professes that sin is an offense against God. What corresponds, in the inscrutable intimacy of the Father, the Word and the Holy Spirit, to this ‘offense,’ this rejection of the Spirit who is love and gift? The concept of God as the necessarily most perfect being certainly excludes from God any pain deriving from deficiencies or wounds; but in the ‘depths of God’ there is a Father’s love that, faced with man’s sin, in the language of the Bible reacts so deeply as to say: ‘I am sorry that I have made him’ (cf. Gen 6:7). (John Paul II. Encyclical Dominun et vivificantem, no.39, May 18, 1986)

Above all, the Council recalls that an essential characteristic of sin is that of being an offense toward God. It is a deed of momentous importance that includes the perverse act of the creature who, knowingly and voluntarily, opposes the will of its Creator and Lord, violating the law of goodness, and entering, through free choice, under the yolk of evil. […] It is necessary to say that it is also an act of betrayal of the divine charity, inasmuch as it is an infraction of the law of friendship and the covenant that God established with his people and with every man through the blood of Christ; and, therefore, an act of infidelity, and, in practice, of rejection of his love. Sin, consequently, is not a simple human error, and does not merely result in a detriment to man: it is an offense toward God, for the sinner violates the law of him, who is Creator and Lord, and offends his fatherly love. One may not consider sin exclusively from the point of view of its psychological consequences: sin acquires its significance from the relation of man with God. (John Paul II. General audience, no. 4, April 15, 1992)

Sin, then, is any transgression in deed, or word, or desire, of the eternal law. And the eternal law is the divine order or will of God, which requires the preservation of natural order, and forbids the breach of it. (Saint Augustine. Reply to Faustus, Book XXII, no. 27)

The sanctity of the soul is violated, which we know to have been wedded to Christ. That temple of the Lord is profaned, against the contaminators of which the Apostle utters this denunciation: ‘If any man violate the temple of God, him shall God destroy’ (1 Cor 3: 16-17). (Catechism of Trent, no. 4500)

God it is against whom, having cast off obedience, we sin; the order of whose wisdom we disturb, as far as in us lies; whom we offend; whom we outrage by words and deeds. (Catechism of Trent, no. 4500)

Innumerable are the evils brought upon man by sin, that almost infinite pest of which David says: ‘There is no health in my flesh, because of thy wrath; there is no peace for my bones, because of my sins.’ In these words he marks the violence of the plague, confessing that it left no part of him uninfected by pestiferous sin; for the poison had penetrated into his bones, that is, it infected his understanding and will, which are the two most intimate faculties of the soul. (Catechism of Trent, no. 4500)

For Christ, while we were still helpless, yet died at the appointed time for the ungodly. Indeed, only with difficulty does one die for a just person, though perhaps for a good person one might even find courage to die. But God proves his love for us in that while we were still sinners Christ died for us. How much more then, since we are now justified by his blood, will we be saved through him from the wrath. Indeed, if, while we were enemies, we were reconciled to God through the death of his Son, how much more, once reconciled, will we be saved by his life! (Rom 5: 6-10)

No, it was not absolutely necessary for Jesus Christ to suffer as much as He did, because each of His acts being of infinite value, the least of His sufferings would have sufficed for our redemption. Jesus Christ suffered so much in order to satisfy divine justice all the more abundantly; to display His love for us still more; and to inspire us with the deepest horror of sin. (Catechism of Saint Pius X, no. 12, 13)

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 the minds of the pious meditate on all these things the more truly, because the sins of men and their crimes committed in every age were the cause why Christ was delivered up to death, and now also they would of themselves bring death to Christ, joined with the same griefs and sorrows, since each several sin in its own way is held to renew the passion of Our Lord: ‘Crucifying again to themselves the Son of God, and making him a mockery’ (Heb 6, 6). (Pius XI. Encyclical Miserentissimus Redemptor, no. 13, May 8, 1928)

The death of Christ, arduous and excruciating, was also a ‘sacrifice of expiation,’ which makes us understand the gravity of sin, a rebellion against God and a rejection of his love, and also the marvelous redeeming work of Christ, who, in expiating for humanity, has restored grace in us, that is, the participation in the same Trinitarian life of God and the inheritance of his eternal happiness. (John Paul II, General audience, no. 3, March 22, 1989)

Jesus Christ, the Incarnation of God, has demonstrated this immense mercy, which takes nothing away from the gravity of sin, but aims always at saving the sinner, at offering him the possibility of redemption, of starting again from the beginning, of converting. (Benedict XVI. Angelus, October 31, 2010)

By their commission [sins] we become guilty before God and incur a debt of punishment, which we must pay either by satisfaction or by suffering. It was of this debt that Christ the Lord spoke by the mouth of His Prophet: ‘Then did I pay that which I took not away’ (Ps 68:5). From these words of God we may understand that we are not only debtors, but also unequal to the payment of our debt, the sinner being of himself utterly incapable of making satisfaction. Wherefore we must fly to the mercy of God; and as justice, of which God is most tenacious, is an equal and corresponding attribute to mercy, we must make use of prayer, and the intercession of the Passion of our Lord Jesus Christ, without which no one ever obtained the pardon of his sins. (Catechism of Trent, 4500)

What then shall we say? Shall we persist in sin that grace may abound? Of course not! How can we who died to sin yet live in it? Or are you unaware that we who were baptized into Christ Jesus were baptized into his death? We were indeed buried with him through baptism into death, so that, just as Christ was raised from the dead by the glory of the Father, we too might live in newness of life. […] We know that our old self was crucified with him, so that our sinful body might be done away with, that we might no longer be in slavery to sin. For a dead person has been absolved from sin. […] Therefore, sin must not reign over your mortal bodies so that you obey their desires. And do not present the parts of your bodies to sin as weapons for wickedness, but present yourselves to God as raised from the dead to life and the parts of your bodies to God as weapons for righteousness. (Rom 6:1-13)

Penance has indeed been necessary for all men, who at any time whatever have stained themselves with mortal sin, in order to attain grace and justice, even for those who have desired to be cleansed by the sacrament of baptism, so that their perversity being renounced and amended, they might detest so great an offense against God with a hatred of sin and a sincere sorrow of heart. Therefore, the Prophet says: ‘Be converted and do penance for all your iniquities; and iniquity shall not be your ruin’ (Ez 18,30). The Lord also said: ‘Except you do penance, you shall all likewise perish’ (Lc 13,3). (Denzinger-Hünermann 1669. Council of Trent, Session XIV, November 25, 1551)

Contrition, which has the first place among the aforementioned acts of the penitent, is a sorrow of the soul and a detestation of sin committed, with a determination of not sinning in the future. This feeling of contrition is, moreover, necessary at all times to obtain the forgiveness of sins, and thus for a person who has fallen after baptism it especially prepares for the remission of sins, if it is united with trust in divine mercy and with the desire of performing the other things required to receive this sacrament correctly. The holy Synod, therefore, declares that this contrition includes not only cessation from sin and a resolution and a beginning of a new life, but also hatred of the old, according to this statement: ‘Cast away from you all your transgressions, by which you have transgressed, and make to yourselves a new heart and a new spirit’ (Ez 18,31). (Denzinger-Hünermann 1676. Council of Trent, Session XIV, November 25, 1551)

We can in no way arrive by the sacrament of penance without many tears and labors on our part, for divine justice demands this, so that penance has justly been called by the holy Fathers, ‘a laborious kind of baptism.’ (Denzinger-Hünermann 1672. Council of Trent, Session XIV, November 25, 1551)

The sinner who beseeches God the remission of his sins cannot but hear this opportune response: ‘Blessed are the merciful, for they shall obtain mercy’ (Mt 5:7). If then you desire that God have compassion on you, have compassion on your soul. Flood your bed every night with tears; and drench your couch with weeping (Ps 6:7). If you have compassion on yourself, if you produce profound cries of penance, you have already made the first step to the plane of mercy, and with all certainty you shall obtain it. (Saint Bernard of Clarirvaux, Of conversion: a sermon to the clergy, ch. XIV, n. 29)

Whereas sin induces such a blot, that not even with ten thousand fountains could one purge it away, but with tears only, and with confessions. But no one hath any sense of this blot. (Saint John Chrysostom, Homily XXXVII of the Gospel of Matthew, no. 8)

To excite myself to detest my sins: (1) I will consider the rigour of the infinite justice of God And the foulness of sin which has defiled my soul and made me worthy of the eternal punishment of hell; (2) I will consider that by sin I have lost the grace, friendship and sonship of God and the inheritance of Heaven; (3) That I have offended my Redeemer who died for me And that my sins caused His death; (4) That I have despised my Creator and my God, that I have turned my back upon Him who is my Supreme Good and worthy of being loved above everything else And of being faithfully served. (Catechism of Saint Pius X, no. 54)

Since, however, to obtain what we ask we must pray in a becoming manner, it appears expedient to explain the disposition with which this prayer should be offered to God. […] he who comes to offer this Petition must first acknowledge, and next feel sorrow and compunction for his sins. He must also be firmly convinced that to sinners, thus disposed and prepared, God is willing to grant pardon […] we ought to be so disposed, that, acknowledging our sins in the bitterness of our souls, we may fly to God as to a Father, not as to a Judge, imploring Him to deal with us not according to His justice, but according to His mercy. (Catechism of Trent, no. 4500)

But moreover, 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. […] God is light, and in Him is no darkness at all. If we say that we have fellowship with Him, and walk in darkness, we lie, and do not the truth: if we walk in the light, as He is in the light, we have fellowship one with another […] But sins are darkness, as the Apostle says of the devil and his angels, that they are ‘rulers of this darkness.’ Ephesians 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. Homily I on the First Epistle of John, no. 5)

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. Accordingly, what follows? ‘My little children, these things I write unto you, that you sin not.’ 1 John 2:1 […] He then is the advocate; do your endeavor not to sin: 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. Homily I on the First Epistle of John, no. 7)

Before all, confession: lest any think himself righteous, and, before the eyes of God who sees that which is, man, that was not and is, lift up the neck. Before all, then, confession; then, love. (Saint Augustine of Hippo. Homily I on the First Epistle of John, no. 6)

There are no limits to the mercy of God, but anyone who deliberately refuses to accept his mercy by repenting, rejects the forgiveness of his sins and the salvation offered by the Holy Spirit (cf. John Paul II, DeV 46). Such hardness of heart can lead to final impenitence and eternal loss. (Catechism of the Catholic Church, no. 1864)

However, to the ‘return’ of God who forgives must correspond the ‘return’, that is, the ‘conversion’, of the one who repents. In fact, the Psalm says that peace and salvation are offered ‘to those who turn to him in their hearts’ (cf. Ps 84 v. 9). Those who set out with determination on the path of holiness receive the gifts of joy, freedom and peace. (John Paul II. General audience, no. 3, September 25, 2002)

By experiencing the tenderness and pardon of the Lord, the penitent is more easily led to acknowledge the gravity of sin, is more resolved to avoid it in order to remain and grow in renewed friendship with him. (Benedict XVI. Address to the Confessors who serve in the four Papal Basilicas of Rome, February 19, 2007)

Therefore, impelled by love and by the wish to placate God for the offenses against His sanctity and His justice and, at the same time, moved by trust in His infinite mercy, we must bear the sufferings of the spirit and of the body that we may expiate our sins and those of our fellow beings and so avoid the twofold penalty of ‘harm’ and of ‘sense’, that is to say, the loss of God–the supreme good–and eternal fire (cf. Mt25: 41; Lumen Gentium, n. 48). (Paul VI. Apostolic exhortation Signum magnum, no. 4, May 13, 1967)

Moreover, if anyone without repentance dies in mortal sin, without a doubt he is tortured forever by the flames of eternal hell. (Denzinger-Hünermann 839. Innocent IV, Letter Sub Catholicae to the Bishop of Tusculum, March 6, 1254)

If we all consider, and understand that more horrifying kind of death, every one who sins dies. But every man is afraid of the death of the flesh; few, of the death of the soul. In regard to the death of the flesh, which must certainly come some time, all are on their guard against its approach: this is the source of all their labor. Man, destined to die, labors to avert his dying; and yet man, destined to live for ever, labors not to cease from sinning. And when he labors to avoid dying, he labors to no purpose, for its only result will be to put off death for a while, not to escape it; but if he refrain from sinning, his toil will cease, and he shall live for ever. (Saint Augustine, Tractate 49 on the Gospel of John, no. 2)

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’ (Rom 2:8-9). 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, no. 4500)

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 sinning without even pain, causes in Him indignation and wrath. (Saint John Chrysostom. Homily XIV on Matthew, no. 5)

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 that, 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 Clarirvaux. Of conversion: a sermon to the clergy, Chap. IV, n. 5)

A grievous kind of death it is, and is distinguished as a habit of wickedness. For it is one thing to fall into sin, another to form the habit of sinning. He who falls into sin, and straightway submits to correction, will be speedily restored to life; for he is not yet entangled in the habit, he is not yet laid in the tomb. But he who has become habituated to sin, is buried, and has it properly said of him, ‘he stinks;’ for his character, like some horrible smell, begins to be of the worst repute. Such are all who are habituated to crime, abandoned in morals. (Saint Augustine of Hippo. Tractate 49 on the Gospel of John, no. 3)

Not until then, assuredly, were either the heavens opened, nor did the Spirit make His approach. Because henceforth He leads us away from the old to the new polity, both opening to us the gates on high, and sending down His Spirit from thence to call us to our country there; and not merely to call us, but also with the greatest mark of dignity. For He has not made us angels and archangels, but He has caused us to become ‘sons of God,’ […] 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. […] 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. […] 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. (Saint John Chrysostom. Homily XII on the Gospel of Matthew, no. 4)

The pastor, however, should not be content with placing before the eyes of the faithful the turpitude of sin. He should also depict the unworthiness and baseness of men, who, though nothing but rottenness and corruption, dare to outrage in a manner beyond all belief the incomprehensible majesty and ineffable excellence of God, particularly after having been created, redeemed and enriched by Him with countless and invaluable benefits. And for what? Only for this, that separating ourselves from God our Father, who is the supreme Good, and lured by the most base rewards of sin, we may devote ourselves to the devil, to become his most wretched slaves. Language is inadequate to depict the cruel tyranny which the devil exercises over those who, having shaken off the sweet yoke of God, and broken the most lovely bond of charity by which our spirit is bound to God our Father, have gone over to their relentless enemy, who is therefore called in Scripture, the prince and ruler of the world, the prince of darkness, and king over all the children of pride. (Catechism of Trent, no. 4500)

Because it offends the holiness and justice of God and scorns God’s personal friendship with man, sin has a twofold consequence. In the first place, if it is grave, it involves deprivation of communion with God and, in consequence, exclusion from a share in eternal life. […] In the second place, ‘every sin, even venial, entails an unhealthy attachment to creatures, which must be purified either here on earth, or after death in the state called Purgatory. This purification frees one from what is called the ‘temporal punishment’ of sin’ (Catechism of the Catholic Church, no. 1472), and this expiation removes whatever impedes full communion with God and with one’s brothers and sisters. (John Paul II. Bull Incarnationis mysterium, no. 10, November 30, 1998)

Sin creates a proclivity to sin; it engenders vice by repetition of the same acts. This results in perverse inclinations which cloud conscience and corrupt the concrete judgment of good and evil. Thus sin tends to reproduce itself and reinforce itself, but it cannot destroy the moral sense at its root. (Catechism of the Catholic Church, no. 1865)

Venial sin weakens charity; it manifests a disordered affection for created goods; it impedes the soul’s progress in the exercise of the virtues and the practice of the moral good; it merits temporal punishment. Deliberate and unrepented venial sin disposes us little by little to commit mortal sin. However venial sin does not set us in direct opposition to the will and friendship of God; it does not break the covenant with God. With God’s grace it is humanly reparable. (Catechism of the Catholic Church, no. 1863)

A man, so long as he bears the flesh, cannot but have some at any rate light sins. But these which we call light, do not make light of. If you make light of them when you weigh them, be afraid when you count them. Many light make one huge sin: many drops fill the river; many grains make the lump. (Saint Augustine of Hippo. Homily I on the First Epistle of John, no. 6)

Forgiveness of sin, as stated above (q.86, a.2), is effected by man being united to God from Whom sin separates him in some way. Now this separation is made complete by mortal sin, and incomplete by venial sin: because, by mortal sin, the mind through acting against charity is altogether turned away from God; whereas by venial sin man’s affections are clogged, so that they are slow in tending towards God. Consequently both kinds of sin are taken away by penance, because by both of them man’s will is disordered through turning inordinately to a created good. (Saint Thomas Aquinas. Summa Theologica III, q. 87, a. 1)

Likewise, care will have to be taken not to reduce mortal sin to an act of ‘fundamental option’- as is commonly said today – against God, intending thereby an explicit and formal contempt for God or neighbor. For mortal sin exists also when a person knowingly and willingly, for whatever reason, chooses something gravely disordered. In fact, such a choice already includes contempt for the divine law, a rejection of God’s love for humanity and the whole of creation; the person turns away from God and loses charity. (John Paul II. Apostolic exhortation Reconciliation et paenitentia, no. 17, December 2, 1984)

Mortal sin is a radical possibility of human freedom, as is love itself. It results in the loss of charity and the privation of sanctifying grace, that is, of the state of grace. If it is not redeemed by repentance and God’s forgiveness, it causes exclusion from Christ’s kingdom and the eternal death of hell, for our freedom has the power to make choices for ever, with no turning back. (Catechism of the Catholic Church, no. 1861)

Mortal sin cannot be forgiven without true Penance, to which it belongs to renounce sin, by reason of its being against God, which is common to all mortal sins. (Saint Thomas Aquinas. Summa Theologica III, q. 86, a. 3)

Punishment is proportionate to sin. Now sin comprises two things. First, there is the turning away from the immutable good, which is infinite, wherefore, in this respect, sin is infinite. Secondly, there is the inordinate turning to mutable good. In this respect sin is finite, both because the mutable good itself is finite, and because the movement of turning towards it is finite, since the acts of a creature cannot be infinite. Accordingly, in so far as sin consists in turning away from something, its corresponding punishment is the ‘pain of loss,’ which also is infinite, because it is the loss of the infinite good, i.e. God. But in so far as sin turns inordinately to something, its corresponding punishment is the ‘pain of sense,’ which is also finite. (Saint Thomas Aquinas. Summa Theologica, I-II, q. 87, a. 4)

Duration of punishment corresponds to duration of fault, not indeed as regards the act, but on the part of the stain, for as long as this remains, the debt of punishment remains. But punishment corresponds to fault in the point of severity. And a fault which is irreparable, is such that, of itself, it lasts for ever; wherefore it incurs an everlasting punishment. But it is not infinite as regards the thing it turns to; wherefore, in this respect, it does not incur punishment of infinite quantity. (Saint Thomas Aquinas. Summa Theologica, I-II, q. 87, a. 4)