The second effect of charity is the observance of the divine commandments. Gregory says: “The love of God is never lazy. It does great things if it is there; if it refuses to work it is not love.” So a clear sign of charity is promptness in carrying out the divine precepts. For we see lovers doing great and difficult things for the sake of their beloved (Jn 14:23): “If anyone loves me, he will keep my word.” We should note that whoever keeps the law of divine love fulfils the whole law. Yet the divine commandments are twofold: Some are affirmative, and charity fulfils these, because the fulfilment of the law of commandments is love, by which the commandments are observed. Other commandments are prohibitive; charity also fulfils these, because it does not act perversely, as the Apostle says (1Cor 13). (Saint Thomas Aquinas. The Ten Commandments, Prologue)

But to fulfill this commandment of love perfectly, four things are required. […] The fourth is complete avoidance of sin. For no one can love God when he is living in sin (Mt 6:24): “You cannot serve God and mammon.” So, if you are living in sin, you do not love God. But that man loved God who said (Is 38:3): “Remember how I walked before you faithfully with a perfect heart.” Also Elijah said (1Kings 18:21): “For how long will you go on limping with two opinions?” As a lame person bends this way and that, so a sinner wavers between sinning and seeking God. Therefore the Lord said (Joel 2:12): “Turn to me with all your heart.” (Saint Thomas Aquinas. The Ten Commandments, Prologue)

The kingdom of God consists chiefly in internal acts: but as a consequence all things that are essential to internal acts belong also to the kingdom of God. Thus if the kingdom of God is internal righteousness, peace, and spiritual joy, all external acts that are incompatible with righteousness, peace, and spiritual joy, are in opposition to the kingdom of God; and consequently should be forbidden in the Gospel of the kingdom. On the other hand, those things that are indifferent as regards the aforesaid, for instance, to eat of this or that food, are not part of the kingdom of God; wherefore the Apostle says before the words quoted: “The kingdom of God is not meat and drink.” (Saint Thomas Aquinas. Summa Theologica, I–II, q. 108, a.1)

Seest thou not, how the commandments, so far from coming of cruelty, come rather of abounding mercy? And if on account of these thou callest the Lawgiver grievous, and hard to bear with; […] how the God of the old covenant, whom they call cruel, will be found mild and meek: and He of the new, whom they acknowledged to be good, will be hard and grievous, according to their madness? Whereas we say, that there is but one and the same Legislator of either covenant, who dispensed all meetly, and adapted to the difference of the times the difference between the two systems of law. Therefore neither are the first commandments cruel, nor the second hard and grievous, but all of one and the same providential care. (Saint John Chrysostom. Homilies on the Gospel of Saint Matthew, Homily XVI, no. 8)

Let us then be obedient to His sayings; let us not oppose ourselves, nor be contentious; for first of all, even antecedently to their rewards, these injunctions have their pleasure and profit in themselves. And if to the more part they seem to be burdensome, and the trouble which they cause, great; have it in thy mind that thou art doing it for Christ’s sake, and the pain will be pleasant. For if we maintain this way of reckoning at all times, we shall experience nothing burdensome, but great will be the pleasure we reap from every quarter. (Saint John Chrysostom. Homilies on the Gospel of Saint Matthew, Homily XVI, no. 8)

The conscientious observation of the ten commandments of God and the precepts of the Church (which are nothing but practical specifications of rules of the Gospels) is for every one an unrivaled school of personal discipline, moral education and formation of character, a school that is exacting, but not to excess. A merciful God, who as Legislator, says – Thou must! – also gives by His grace the power to will and to do. To let forces of moral formation of such efficacy lie fallow, or to exclude them positively from public education, would spell religious under-feeding of a nation. To hand over the moral law to man’s subjective opinion, which changes with the times, instead of anchoring it in the holy will of the eternal God and His commandments, is to open wide every door to the forces of destruction. The resulting dereliction of the eternal principles of an objective morality, which educates conscience and ennobles every department and organization of life, is a sin against the destiny of a nation, a sin whose bitter fruit will poison future generations. (Pius XI. Encyclical Mit Brennender Sorge, no. 29)

God wants us to be happy. That is why he gave us specific directions for the journey of life: the commandments. If we observe them, we will find the path to life and happiness. At first glance, they might seem to be a list of prohibitions and an obstacle to our freedom. But if we study them more closely, we see in the light of Christ’s message that the commandments are a set of essential and valuable rules leading to a happy life in accordance with God’s plan. (Benedict XVI. Message for the twenty-seventh World Youth Day, March 15, 2012)

If you wish to enter into life, keep the commandments. (Mt 19:17)

All these curses will come upon you, pursuing you and overwhelming you, until you are destroyed, because you would not hearken to the voice of the Lord, your God, nor keep the commandments and statutes he gave you. They will light on you and your descendants as a sign and a wonder for all time. (Dt 28:45–46)

We must now consider upon whom rests the obligation to dissipate this most pernicious ignorance [of religion] and to impart in its stead the knowledge that is wholly indispensable. There can be no doubt, Venerable Brethren, that this most important duty rests upon all who are pastors of souls. On them, by command of Christ, rest the obligations of knowing and of feeding the flocks committed to their care; and to feed implies, first of all, to teach. “I will give you pastors according to my own heart,” God promised through Jeremias, “and they shall feed you with knowledge and doctrine” (Jer 3:15). […] the first duty of all those who are entrusted in any way with the government of the Church is to instruct the faithful in the things of God. (Pius X. Encyclical Acerbo nimis, no. 7, April 15, 1905)

The commandments of which Jesus reminds the young man are meant to safeguard the good of the person, the image of God, by protecting his goods. ‘You shall not murder; You shall not commit adultery; You shall not steal; You shall not bear false witness’ are moral rules formulated in terms of prohibitions. These negative precepts express with particular force the ever urgent need to protect human life, the communion of persons in marriage, private property, truthfulness and people’s good name. The commandments thus represent the basic condition for love of neighbour; at the same time they are the proof of that love. They are the first necessary step on the journey towards freedom, its starting-point. (John Paul II. Encyclical Veritatis splendor, no 13, August 6, 1993)

The Gospel is absolutely not a code that is easily fulfilled: it demands effort and fidelity. Here one may analyze the moral systems that renounce personal effort to obtain salvation, in the erroneous conviction that it is only by faith and only by grace that we have the fortune of being saved, without a positive and systematic moral discipline; as if faith and grace, gifts of God and true causes of salvation, do not demand a response, coherence, free and responsible cooperation on our part, either as a condition of cooperating in the saving work of God in us, or also as a consequence of the rebirth brought about by his merciful supernatural action. (Paul VI. General audience, July 7, 1971)

As he perceives the spokesmen of these tendencies deny or in practice neglect the vivifying truths and the values inherent in belief in God and in Christ; as he perceives them wantonly break the Tables of God’s Commandments to substitute other tables and other standards stripped of the ethical content of the Revelation on Sinai, standards in which the spirit of the Sermon on the Mount and of the Cross has no place? (Pius XII. Encyclical Summi pontificatus, no. 7, October 20, 1939)

The task of the catechist is to take up one or other of the truths of faith or of Christian morality and then explain it in all its parts; and since amendment of life is the chief aim of his instruction, the catechist must needs make a comparison between what God commands us to do and what is our actual conduct. After this, he will use examples appropriately taken from the Holy Scriptures, Church history, and the lives of the saints – thus moving his hearers and clearly pointing out to them how they are to regulate their own conduct. He should, in conclusion, earnestly exhort all present to dread and avoid vice and to practice virtue. (Pius X. Encyclical Acerbo nimis, no. 13, April 15, 1905)

Nevertheless, the conditions of history and of life are not to be considered the main impediment to human freedom. When man freely applies himself to the work of salvation, he finds sin the greatest obstacle. “Although he was made by God in a state of holiness, from the very dawn of history man abused his liberty, at the urging of the Evil One. Man set himself against God and sought to find fulfilment apart from God” (GS, 13). “Through one man sin entered the world, and with sin death, death thus coming to ail men inasmuch as ail sinned” (Rom 5, 12). “human nature so fallen, stripped of the grace that clothed it, injured in its own natural powers and subjected to the dominion of death, that is transmitted to ail men, and it is in this sense that every man is born in sin” (Paul VI, Profession fidei, n. 16). The multitude of sins, then, has become a sorrowful experience for mankind, and it is also the cause of manifold sorrows and ruin. (Congregation for the Clergy. General catechetical directory, no. 62)

One must not neglect the teaching on the nature and effects of personal sins, whereby man, adding knowingly and deliberately, by his act violates the moral law, and in a serious matter also seriously offends God. (Congregation for the Clergy. General catechetical directory, no. 62)

The history of salvation is also the history of liberation from sin. Every intervention of God both in the old and in the New Testament was to give guidance to men in the struggle against the forces of evil. The role entrusted to Christ in the history of salvation relates to the destruction of sin, and is fulfilled through the mystery of the cross. The profound reflections found in Saint Paul (cf. Rom 5) concerning the reality of sin and Christ’s consequent ‘work of justice’ must be numbered among the principal points of the Christian faith, and it is not right to pass over them in silence in catechesis. (Congregation for the Clergy. General catechetical directory, no. 62)

Christ commissioned his apostles to teach the observance of everything that he had commanded (cf. Mt 28:20). Catechesis, therefore, must include not only those things which are to be believed, but also those things which are to be done. The moral life of Christians, which is a way of adding that is worthy of a man and an adopted son of God, is a response to the duty of living and growing, under the guidance of the Holy Spirit, in the new life communicated through Jesus Christ. The moral life of Christians is guided by the grace and gifts of the Holy Spirit. ‘The love of God has been poured out in our hearts through the Holy Spirit who has been given to us’ (Rom 5:5). The docility with which the Holy Spirit must be obeyed entails a faithful observance of the commandments of God, the laws of the Church, and just civil laws. (Congregation for the Clergy. General catechetical directory, no. 62)

Immorality or any impurity or greed must not even be mentioned among you, as is fitting among holy ones, no obscenity or silly or suggestive talk, which is out of place, but instead, thanksgiving. Watch carefully then how you live, not as foolish persons but as wise. Therefore, do not continue in ignorance, but try to understand what is the will of the Lord. (Eph 5:3, 4, 15, 17)

All the people who listened, including the tax collectors, and who were baptized with the baptism of John, acknowledged the righteousness of God; but the Pharisees and scholars of the law, who were not baptized by him, rejected the plan of God for themselves. “Then to what shall I compare the people of this generation? What are they like? They are like children who sit in the marketplace and call to one another, ‘We played the flute for you, but you did not dance. We sang a dirge, but you did not weep.’ For John the Baptist came neither eating food nor drinking wine, and you said, ‘He is possessed by a demon.’ The Son of Man came eating and drinking and you said, ‘Look, he is a glutton and a drunkard, a friend of tax collectors and sinners.’ But wisdom is vindicated by all her children.” (Lk 7:29–35)

In fact, all who want to live religiously in Christ Jesus will be persecuted. (2Tim 3:12)

Behold, I am sending you like sheep in the midst of wolves; so be shrewd as serpents and simple as doves. But beware of people, for they will hand you over to courts and scourge you in their synagogues, and you will be led before governors and kings for my sake as a witness before them and the pagans. (Mt 10:16–18)

I gave them your word, and the world hated them, because they do not belong to the world any more than I belong to the world. (Jn 17:14)

Many centuries have gone by since Christ. The heredity of God has been growing marvelously, not without repetition of the rejection, the incomprehension and the struggles – over the corner stone: Christ dead and resurrected. (John Paul II. Homily in Saint Bartholomew of Orcasitas Church, no. 2, November 3, 1982)

For it cannot, it cannot be, that one careful of virtue, should not have many enemies. However, this is nothing to the virtuous man. For by such means his brightness will increase the more abundantly. Let us then, bearing these things in mind, look to one object only; how to order our own life with strictness. For thus we shall also guide to the life that is there, such as are now sitting in darkness. (Saint John Chrysostom. Homilies on the Gospel of Saint Matthew, Homily XV, no. 12)

This petition having been made, hear what Jesus answers them: ‘You do not know what you are asking for’. For it is not the time for crowns or prizes; it is the time for struggles, conflicts, toils, sweat, trials, and battles. […] This, then, is the meaning of the words “You do not know what you are asking for”. You have not yet experienced imprisonment, you have not yet gone out to proclaim and combat. ‘Can you drink of the chalice that I drink of: or be baptized with the baptism wherewith I am baptized?’ (Mk 10: 38). In this passage he calls his cross and death his chalice and baptism: chalice, for the eagerness with which he drinks of it; baptism, because by means of his death he would purify the whole world; and not only did he redeem it in this way, but by means of the resurrection, although this was not onerous to him. […] He says to them: You will drink of the chalice that I will drink of and be baptized with the baptism wherewith I will be baptized, referring, in this way to death. Effectively, Saint James was decapitated, and John was condemned to death several times. […] You will surely die, they will kill you, you will achieve the crown of martyrdom; but as for being the first, it is not for me to give: those who fight will receive it, by means of their greater effort, in accordance to their greater readiness of spirit. (Saint John Chrysostom. Homily 8 Against the Anomoeans – The petition of the mother of the sons of Zebedee)

However, the just keep their fidelity intact: “I have sworn and have made up my mind to obey your decrees… I remember your law… I do not stray from your precepts” (Ps 119[118]: 106, 109, 110). A conscience at peace is the strength of believers; their constancy in obeying the divine commandments is the source of their serenity. The final declaration is therefore consistent: “Your will is my heritage for ever, the joy of my heart” (v. 111) It is this that is the most precious reality, the “heritage”, the “reward” (cf. v. 112) which the Psalmist cherishes with vigilant and ardent love: the teaching and commandments of the Lord. He wants to be totally faithful to the will of his God. On this path he will find peace of soul and will succeed in getting through the dark tangle of trials and reaching true joy. (John Paul II. General audience, no. 3, June 21, 2004)

The theology of the Cross is not a theory it is the reality of Christian life. To live in the belief in Jesus Christ, to live in truth and love implies daily sacrifice, implies suffering. Christianity is not the easy road, it is, rather, a difficult climb, but one illuminated by the light of Christ and by the great hope that is born of him. Saint Augustine says: Christians are not spared suffering, indeed they must suffer a little more, because to live the faith expresses the courage to face in greater depth the problems that life and history present. But only in this way, through the experience of suffering, can we know life in its profundity, in its beauty, in the great hope born from Christ crucified and risen again. (Benedict XVI. General audience, November 5, 2008)

Faith involves a change of life, a ‘metanoia’, that is a profound transformation of mind and heart; it causes the believer to live that conversion. This transformation of life manifests itself at all levels of the Christian’s existence: in his interior life of adoration and acceptance of the divine will, in his action, participation in the mission of the Church, in his married and family life; in his professional life; in fulfilling economic and social responsibilities. Faith and conversion arise from the ‘heart’, that is, they arise from the depth of the human person and they involve all that he is. By meeting Jesus Christ and by adhering to him the human being sees all of his deepest aspirations completely fulfilled. (Congregation for the Clergy. General Directory for Catechesis, no. 55)

From its commencement, dearly beloved, does this great Psalm exhort us unto bliss, which there is no one who desires not. And therefore this is the lesson which he teaches, who says, “Blessed are those that are undefiled in the way, who walk in the law of the Lord” (Ps 118:1). As much as to say, I know what you wish, you are seeking bliss: if then you would be blessed, be undefiled. For the former all desire, the latter fear: yet without it what all wish cannot be attained. But where will any one be undefiled, save in the way? In what way, save in the law of the Lord? […] No other class of the blessed seems to me to be mentioned in these words, than that which has been already spoken of. For to examine into the testimonies of the Lord, and to seek Him with all the heart, this is to be undefiled in the way, this is to walk in the law of the Lord. (Saint Augustine of Hippo. Ennarations on the Psalms, Psalm 119)

I answer that, The Holy Ghost dwells in us by charity, as shown above (STh, II-II a.2; qq. 23,24). We can, accordingly, consider charity in three ways: first on the part of the Holy Ghost, Who moves the soul to love God, and in this respect charity is incompatible with sin through the power of the Holy Ghost, Who does unfailingly whatever He wills to do. Hence it is impossible for these two things to be true at the same time – that the Holy Ghost should will to move a certain man to an act of charity, and that this man, by sinning, should lose charity. For the gift of perseverance is reckoned among the blessings of God whereby “whoever is delivered, is most certainly delivered,” as Augustine says in his book on the Predestination of the saints (De Dono Persev. XIV). Secondly, charity may be considered as such, and thus it is incapable of anything that is against its nature. Wherefore charity cannot sin at all, even as neither can heat cool, nor unrighteousness do good, as Augustine says (De Serm. Dom. in Monte II, 24). Thirdly, charity can be considered on the part of its subject, which is changeable on account of the free-will. Moreover charity may be compared with this subject, both from the general point of view of form in comparison with matter, and from the specific point of view of habit as compared with power. Now it is natural for a form to be in its subject in such a way that it can be lost, when it does not entirely fill the potentiality of matter: this is evident in the forms of things generated and corrupted, because the matter of such things receives one form in such a way, that it retains the potentiality to another form, as though its potentiality were not completely satisfied with the one form. Hence the one form may be lost by the other being received. On the other hand the form of a celestial body which entirely fills the potentiality of its matter, so that the latter does not retain the potentiality to another form, is in its subject inseparably. Accordingly the charity of the blessed, because it entirely fills the potentiality of the rational mind, since every actual movement of that mind is directed to God, is possessed by its subject inseparably: whereas the charity of the wayfarer does not so fill the potentiality of its subject, because the latter is not always actually directed to God: so that when it is not actually directed to God, something may occur whereby charity is lost. It is proper to a habit to incline a power to act, and this belongs to a habit, in so far as it makes whatever is suitable to it, to seem good, and whatever is unsuitable, to seem evil. For as the taste judges of savors according to its disposition, even so does the human mind judge of things to be done, according to its habitual disposition. Hence the Philosopher says (Ethic. iii, 5) that “such as a man is, so does the end appear to him.” Accordingly charity is inseparable from its possessor, where that which pertains to charity cannot appear otherwise than good, and that is in heaven, where God is seen in His Essence, which is the very essence of goodness. Therefore the charity of heaven cannot be lost, whereas the charity of the way can, because in this state God is not seen in His Essence, which is the essence of goodness.

Replies to objections:
ad 1: The passage quoted speaks from the point of view of the power of the Holy Ghost, by Whose safeguarding, those whom He wills to move are rendered immune from sin, as much as He wills.
ad 2: The charity which can fail by reason of itself is no true charity; for this would be the case, were its love given only for a time, and afterwards were to cease, which would be inconsistent with true love. If, however, charity be lost through the changeableness of the subject, and against the purpose of charity included in its act, this is not contrary to true charity.
ad 3: The love of God ever works great things in its purpose, which is essential to charity; but it does not always work great things in its act, on account of the condition of its subject.
ad 4: Charity by reason of its act excludes every motive for sinning. But it happens sometimes that charity is not acting actually, and then it is possible for a motive to intervene for sinning, and if we consent to this motive, we lose charity. (Saint Thomas Aquinas. Summa Theologica II-II, q. 24, a.11)

Objection 5: Further, the object of a theological virtue is the last end. Now the other theological virtues, namely faith and hope, are not done away by one mortal sin, in fact they remain though lifeless. Therefore [it would seem that] charity can remain without a form, even when a mortal sin has been committed.
On the contrary, By mortal sin man becomes deserving of eternal death, according to Rm 6:23: “The wages of sin is death.” On the other hand whoever has charity is deserving of eternal life, for it is written (Jn. 14:21): “He that loveth Me, shall be loved by My Father: and I will love Him, and will manifest Myself to him,” in which manifestation everlasting life consists, according to Jn. 17:3: “This is eternal life; that they may know Thee the . . . true God, and Jesus Christ Whom Thou hast sent.” Now no man can be worthy, at the same time, of eternal life and of eternal death. Therefore it is impossible for a man to have charity with a mortal sin. Therefore charity is destroyed by one mortal sin.
I answer that, that one contrary is removed by the other contrary supervening. Now every mortal sin is contrary to charity by its very nature, which consists in man’s loving God above all things, and subjecting himself to Him entirely, by referring all that is his to God. It is therefore essential to charity that man should so love God as to wish to submit to Him in all things, and always to follow the rule of His commandments; since whatever is contrary to His commandments is manifestly contrary to charity, and therefore by its very nature is capable of destroying charity. If indeed charity were an acquired habit dependent on the power of its subject, it would not necessarily be removed by one mortal sin, for act is directly contrary, not to habit but to act. Now the endurance of a habit in its subject does not require the endurance of its act, so that when a contrary act supervenes the acquired habit is not at once done away. But charity, being an infused habit, depends on the action of God Who infuses it, Who stands in relation to the infusion and safekeeping of charity, as the sun does to the diffusion of light in the air, as stated above (a. 10, ad 3). Consequently, just as the light would cease at once in the air, were an obstacle placed to its being lit up by the sun, even so charity ceases at once to be in the soul through the placing of an obstacle to the outpouring of charity by God into the soul. Now it is evident that through every mortal sin which is contrary to God’s commandments, an obstacle is placed to the outpouring of charity, since from the very fact that a man chooses to prefer sin to God’s friendship, which requires that we should obey His will, it follows that the habit of charity is lost at once through one mortal sin. Hence Augustine says (Gen. ad lit. 8, 12) that “man is enlightened by God’s presence, but he is darkened at once by God’s absence, because distance from Him is effected not by change of place but by aversion of the will.”
Reply to Objection 5: Charity denotes union with God, whereas faith and hope do not. Now every mortal sin consists in aversion from God, as stated above (Gen. ad lit. 8, 12). Consequently every moral sin is contrary to charity, but not to faith and hope, but only certain determinate sins, which destroy the habit of faith or of hope, even as charity is destroyed by every moral sin. Hence it is evident that charity cannot remain lifeless, since it is itself the ultimate form regarding God under the aspect of last end as stated above (Saint Thomas Aquinas. Summa Theologica II-II, q. 24, a.12)