1. The ten commandments of the Decalogue
Our Lord Jesus Christ taught that to be saved we must carry out the commandments. When a young man asked him: Good Master, what good deed must I do, to have eternal life? (Mt 19:16), Jesus answered: If you would enter into life, keep the commandments (Mt 19:17). He went on to mention several precepts referring to love for our neighbor: You shall not kill, You shall not commit adultery, You shall not steal, You shall not bear false witness, Honor your father and mother (Mt 19:18-19). These precepts, along with those referring to love for God that our Lord mentions on other occasions, make up the ten commandments of the divine Law (cf. Ex 20: 1-17; Catechism , 2052). “The first three concern love of God, and the other seven the love of neighbor" (Catechism, 2067).
The ten commandments express the substance of the natural moral law (cf. Catechism, 1955). This is a law written on the human heart, but our knowledge of it has become blurred as a consequence of original sin and repeated personal sins. God wanted to reveal “religious and moral truths that of themselves are not beyond the grasp of human reason" (Catechism, 38), so that everyone can have full and certain knowledge of them (cf. Catechism, 37-38). He first revealed these truths in the Old Testament and later on, in a full way, through Jesus Christ (cf. Catechism, 2053-2054). The Church safeguards Revelation and teaches it to all men (cf. Catechism, 2071).
Some commandments determine what must be done (e.g. keep feast days); others point out what it is never permitted to do (e.g. kill an innocent person). The latter indicate certain acts that are intrinsically evil because of their moral object, regardless of the motives or ulterior intentions of those who carry them out and the circumstances surrounding them. 
“Jesus shows that the commandments should not be understood merely as an ultimate limit not to be transgressed but rather as a path leading to a way of moral and spiritual perfection, whose inner impulse is love (cf. Col 3:14)."  For example, the commandment “You shall not kill" contains the call not only to respect the life of our neighbors but also to further their development and foster their enrichment as persons. The commandments should be seen not as prohibitions that restrict our freedom, but as lights that indicate the path to human fullness and happiness, freeing us from moral error.
2. The first commandment
The first commandment is twofold: love for God and love for neighbor out of love for God. 'Teacher, which is the great commandment in the law?' And he said to him, 'You shall love the Lord your God with all your heart, and with all your soul, and with all your mind. This is the great and first commandment. And a second is like it, You shall love your neighbor as yourself. On these two commandments depend all the law and the prophets' (Mt 22: 36-40).
This love is called charity. The same term is also used for the theological virtue of charity, which entails love for God and love for our neighbor because of God. Charity is a gift that the Holy Spirit infuses in those who have become adopted children of God (cf. Rom 5:5). Charity has to grow throughout our life on earth through the action of the Holy Spirit and our own cooperation: growing in holiness is growing in charity. Holiness is nothing other than the fullness of divine filiation and charity. It can also diminish through venial sin and even be lost through grave sin. There is an order in charity: God, others (out of love for God) and oneself (out of love for God).
Loving God as his children requires:
a) choosing him as the ultimate end of all we undertake. We are asked to do everything out of love for him and for his glory: whether you eat or drink, or do anything else, do all for the glory of God (1 Cor 10:31). “ Deo omnis gloria . All glory to God."  No love can be put higher than love for God. He who loves father or mother more than me is not worthy of me; and he who loves son or daughter more than me is not worthy of me ( Mt 10:37). “The only real love is God's love!"  Any love that tries to exclude God or put him in second place is not a true love.
b) carrying out God's will in deeds: Not everyone who says to me, 'Lord, Lord,' shall enter the kingdom of heaven, but he who does the will of my Father who is in heaven " (Mt 7:21). God's will is that we be saints (cf. 1 Thess 4:3), that we follow Christ (cf. Mt 17:5), carrying out his commandments (cf. Jn 14:21). “Do you really want to be a saint? Carry out the little duty of each moment; do what you ought, and put yourself in what you are doing."  We need to do so also when it calls for sacrifice: not my will but thine be done (Lk 22:42).
c) corresponding to his love for us. He loved us first; he created us as free beings and has made us his children (cf. 1 Jn 4:19). Sin is refusing God's love (cf. Catechism, 2094). But God is always ready to pardon us, and he gives himself up for us. In this is love, not that we loved God but that he loved us and sent his Son to be the expiation for our sins (1 Jn 4:10; cf. Jn 3:16). He loved me and gave himself up for me (Gal 2:20). “To correspond to so much love requires of us a total gift, body and soul."  This is not a matter of feeling but rather a decision of the will which may or may not be accompanied by our emotions.
Loving God leads us to seek a personal relationship with him, a relationship consisting of prayer, which in turn fosters love. Prayer can take several forms  :
a) “ Adoration is the first attitude of man acknowledging that he is a creature before his Creator" (Catechism, 2628). It is the most fundamental attitude of religion (cf. Catechism , 2096). You shall worship the Lord your God and him only shall you serve (Mt 4:10). Adoring God frees us from the various forms of idolatry that lead to slavery. “May your prayer always be a real and sincere act of adoration of God." 
b) Thanksgiving (cf. Catechism 2638), because everything we are and have we have received from him: What have you that you did not receive? If then you received it, why do you boast as if it were not a gift? (1 Cor 4:7).
c) Petition : asking for pardon for what separates us from God (sin) and asking for help for ourselves and for others, as well as for the Church and all mankind. These two forms of petition are found in the Our Father: “… give us this day our daily bread, and forgive us our trespasses." A Christian's petition is full of confidence, for in hope were we saved " (Rom 8:24), and because it is a filial petition through Christ: if you ask the Father anything in my name, he will give it to you " (Jn 16:23; cf. 1 Jn 5:14-15).
Love is also shown by sacrifice, closely tied to prayer: “prayer is worth more with sacrifice."  Sacrifice is offering God something that costs us, in homage to him and as an expression of the interior surrender of our own will, that is, of obedience to God. Christ redeemed us through the sacrifice of the Cross, showing his perfect obedience to the point of dying for us (cf. Phil 2:8). As Christians, as members of Christ, we can coredeem with him, uniting our sacrifices to his, in Holy Mass (cf. Catechism , 2100).
The worship of God consists of prayer and sacrifice. It is called the worship of latria or adoration, to distinguish it from the cult due to the angels and saints, which is the worship of dulia or veneration, and the cult honoring the Blessed Virgin, called hyperdulia (cf. Catechism , 971). The pre-eminent act of worship is Holy Mass, image of the heavenly liturgy. The love of God should be reflected by the dignity of our worship: observing what the Church prescribes, with “the good manners of piety,"  cleanliness and care for liturgical objects. “That woman in the house of Simon the leper in Bethany, who anoints the Master's head with precious ointment, reminds us of the duty to be generous in the worship of God. All the richness, majesty and beauty seem too little to me." 
3. Faith and hope in God
Faith, hope and charity are the three “theological" virtues (virtues directed to God). The greatest is charity (cf. 1 Cor 13:13), which gives “form" and supernatural “life" to faith and hope (in a way analogous to how the soul gives life to the body). But charity in this life presupposes faith because only those who know God can love him; and it also presupposes hope, because only those who make their desire for happiness depend on God can love him.
Faith is a gift from God, a light in our intellect that enables us to know the truth that God has revealed and to assent to it. This implies two things: believing what God has revealed (the mystery of the Holy Trinity and all the articles of the Creed) and believing that it is God himself who has revealed it (trusting in him). There is no opposition—nor can there ever be—between faith and reason.
Doctrinal formation is important for attaining a solid faith, and therefore for fostering love for God and others out of love for God: for sanctity and apostolate. The life of faith is a life based on faith and consistent with it.
Hope is a gift from God that leads us to desire union with him, where we will find happiness. It gives us confidence that he will give us the capacity and the means to attain it (cf. Catechism , 2090).
Christians should always be rejoicing in hope (Rom 12:12), because if we are faithful the happiness of heaven awaits us, the vision of God face to face (1 Cor 13:12), the beatific vision. If we are sons, we are heirs also: heirs indeed of God and joint heirs with Christ, provided, however, we suffer with him that we may also be glorified with him " (Rom 8:17) Christian life in this world is a path of happiness because even now through grace we have a pledge of this union with the Holy Trinity; but it is a happiness mixed with suffering, with the Cross. Hope makes us aware that it is all worthwhile: “It is worthwhile putting our whole life at stake, working and suffering for Love, in order to accomplish God's plans and coredeem with him." 
Sins against the first commandment are sins against the theological virtues:
a) Against faith: atheism, agnosticism, religious indifference, heresy, apostasy, schism, etc. (cf. Catechism 2089) It is also contrary to the first commandment to put in danger voluntarily one's own faith, either through reading books against the faith or morals without a sufficient reason and preparation, or through failing to employ the means required to safeguard one's faith.
b) Against hope: despairing of one's own salvation (cf. Catechism 2091) and, at the opposite extreme, presuming that divine mercy will pardon sins without conversion or contrition, or without the necessary sacrament of penance (cf. Catechism 2092). It is also against this virtue to put one's hope for final happiness in anything outside of God.
c) Against charity: any sin whatsoever goes against charity. Directly contrary to it is the rejection of God, and also lukewarmness—not wanting to love him with all one's heart. Opposed to the worship of God is sacrilege, simony, certain superstitious practices, magic, etc., and Satanism (cf. Catechism 2111-2128).
4. Love for others out of love for God
Love for God requires loving those God loves. If anyone says, 'I love God,' and hates his brother, he is a liar; for he who does not love his brother whom he has seen, cannot love God whom he has not seen. And this commandment we have from him, that he who loves God should love his brother also (1 Jn 4: 20-21). We cannot love God without loving all men and women, created by him in his image and likeness, and called to be his children by divine grace (cf. Catechism 2069).
We have to behave as God's children with all God's sons and daughters: 
a) behaving like a child of God, like another Christ. The standard for loving others is Christ's love: A new commandment I give you, that you love one another; even as I have loved you, that you also love one another. By this will all men will know that you are my disciples (Jn 13:34-35). The Holy Spirit has been sent into our hearts so that we can love as God's children, with Christ's love (cf. Rom 5:5). “We must give our life for others. That is the only way to live the life of Jesus Christ and to become one and the same with Him." 
b) seeing a child of God, Christ, in others: As you did it to one of the least of my brethren, you did it to me (Mt 25:40). We need to want for others what God wants, their real good: for them to be saints, and so to be happy. The first manifestation of charity is apostolate. This also includes caring for others' material needs. It means being understanding (“making one's own") the spiritual and material needs of others; and being merciful (cf. Mt 18:15). Love is patient and kind; love is not jealous … does not insist on its own way; it is not irritable or resentful (1 Cor 13:4-5). It also requires fraternal correction (cf Mt 18:15).
5. Love of self out of love for God
The precept of charity also includes love for oneself: You shall love your neighbour as yourself (Mt 22:39) This rightly ordered love of self means loving ourselves out of love for God. It leads to seeking for ourselves what God wants: sanctity and, thereby, happiness (with sacrifice on this earth, with the Cross). There is also a disordered love of self, egoism, which is loving ourselves for our own excellence, and not for love of God. This means putting our own will before God's, and our own interests before serving others.
It is impossible to have a rightly ordered love of self without struggling against selfishness. This involves denying oneself, giving oneself to God and others. If any man would come after me, let him deny himself and take up his cross and follow me. For whoever would save his life will lose it, and whoever loses his life for my sake will find it (Mt 16:24-25). Man “cannot fully find himself except through a sincere gift of himself." 
Catechism of the Catholic Church , 2064-2132.
Benedict XVI, Enc. Deus caritas est , 25 December 2005, 1-18.
Benedict XVI, Enc. Spe salvi , 30 November 2007.
St Josemaria, “Living by Faith", “The Christian's Hope", “The Strength of Love" in Friends of God , 190-237.
 Cf. John Paul II, Enc. Veritatis splendor, 6 August 1993, 80.
 Ibid ., 15.
 St Josemaría, The Way , 780.
 Ibid. , 417.
 Ibid. , 815. Cf. Ibid. , 933.
 St Josemaría, Christ is Passing By , 87.
 Cf. St Josemaría, The Way , 91.
 St Josemaría, The Forge , 263.
 St Josemaría, The Way , 81.
 Cf. ibid., 541.
 Ibid . 527. Cf. Mt 26: 6-13 .
 St Josemaría, The Forge , 26.
 Cf. St Josemaría, Christ is Passing By , 36.
 St Josemaría, The Way of the Cross, XIVth Station. Cf. Benedict XVI, Enc. Deus Caritas est , 25 August 2005, 12-15.
 Vatican Council II, Gaudium et spes , 24.