We are all born with the effects of Original Sin in our souls which leave their mark by a proclivity to sin. We can see that most clearly in our disordered desires. Some examples would be: desiring junk food, over-eating, over-sleeping, over-drinking, spending too much money on frivolous things, gossip, base humor, feelings of superiority, judging other people, need for attention and admiration, lust, etc. All of this is further compounded by our personal sins which lead to sinful habits.
As we cooperate with the grace we receive from the Mass and the sacraments - especially in the Sacrament of Reconciliation & in receiving the Sacred Heart of Jesus in Holy Communion (on a regular basis, in the state of grace, with an open heart, & a desire to offer ourselves completely in, with & through Christ to the Father for the salvation of the world) our disordered, worldly desires slowly but surely - gradually - change into God’s desires. Eventually, we don’t like sin any more and we desire holiness and goodness.
Eventually, our good desires become stronger than our disordered desires and it is our good desires that draw God’s grace into our souls more fully.
But...even the graces from Holy Mass and the sacraments can’t be enacted without the regular (daily) practice of mental prayer. Mental prayer - Christian Meditation - prepares our hearts for a fruitful reception of Holy Communion and it allows the grace we receive in Holy Communion to work more efficiently - to change our desires and to empower us to act on our good desires. It actualizes the grace we have received. It stirs it up so to speak. (see methods of mental prayer here)
How does this happen? Spending time with God in prayer allows Him to work on our hearts & desires directly - in ways that are not detectable to our senses - but it also allows Him to work on our hearts and desires in ways that are detectable by showing us our disordered desires so that we can work on them together. In prayer God inspires us to make resolutions to practice the virtues that will change our hearts and our desires. As we go about our day, his grace helps us to complete the resolutions we have made and when we fail, it helps us to get back up and try again without giving into discouragement.
One thing we should keep in mind, however, is that we never completely lose our sinful desires while we exist on this side of death. But, we can make great strides in this battle, strides that exceed by far what the human will can accomplish on its own. We can become heroically virtuous people due to the effects of grace, especially the grace that we receive from Holy Communion combined with mental prayer. Struggling to fight a sinful, disordered desire is very pleasing to God even if for the rest of our life we have to struggle with it - fighting manfully to overcome it. This struggle can be the very means for a deeper union with God as St. Paul taught us :
“Therefore, in order to keep me from becoming conceited, I was given a thorn in my flesh, a messenger of Satan, to torment me. Three times I pleaded with the Lord to take it away from me. But he said to me, ‘My grace is sufficient for you, for my power is made perfect in weakness.’ Therefore I will boast all the more gladly about my weaknesses, so that Christ’s power may rest on me” (2 Cor. 12: 7-9).
The Fruits of Holy Communion
1391 Holy Communion augments our union with Christ. The principal fruit of receiving the Eucharist in Holy Communion is an intimate union with Christ Jesus. Indeed, the Lord said: "He who eats my flesh and drinks my blood abides in me, and I in him." 226 Life in Christ has its foundation in the Eucharistic banquet: "As the living Father sent me, and I live because of the Father, so he who eats me will live because of me." 227
On the feasts of the Lord, when the faithful receive the Body of the Son, they proclaim to one another the Good News that the first fruits of life have been given, as when the angel said to Mary Magdalene, "Christ is risen!" Now too are life and resurrection conferred on whoever receives Christ. 228
1392 What material food produces in our bodily life, Holy Communion wonderfully achieves in our spiritual life. Communion with the flesh of the risen Christ, a flesh "given life and giving life through the Holy Spirit," 229 preserves, increases, and renews the life of grace received at Baptism. This growth in Christian life needs the nourishment of Eucharistic Communion, the bread for our pilgrimage until the moment of death, when it will be given to us as viaticum.
1393 Holy Communion separates us from sin. The body of Christ we receive in Holy Communion is "given up for us," and the blood we drink "shed for the many for the forgiveness of sins." For this reason the Eucharist cannot unite us to Christ without at the same time cleansing us from past sins and preserving us from future sins:
For as often as we eat this bread and drink the cup, we proclaim the death of the Lord. If we proclaim the Lord's death, we proclaim the forgiveness of sins. If, as often as his blood is poured out, it is poured for the forgiveness of sins, I should always receive it, so that it may always forgive my sins. Because I always sin, I should always have a remedy. 230
1394 As bodily nourishment restores lost strength, so the Eucharist strengthens our charity, which tends to be weakened in daily life; and this living charity wipes away venial sins. 231 By giving himself to us Christ revives our love and enables us to break our disordered attachments to creatures and root ourselves in him:
Since Christ died for us out of love, when we celebrate the memorial of his death at the moment of sacrifice we ask that love may be granted to us by the coming of the Holy Spirit. We humbly pray that in the strength of this love by which Christ willed to die for us, we, by receiving the gift of the Holy Spirit, may be able to consider the world as crucified for us, and to be ourselves as crucified to the world.... Having received the gift of love, let us die to sin and live for God. 232
1395 By the same charity that it enkindles in us, the Eucharist preserves us from future mortal sins. The more we share the life of Christ and progress in his friendship, the more difficult it is to break away from him by mortal sin. The Eucharist is not ordered to the forgiveness of mortal sins - that is proper to the sacrament of Reconciliation. The Eucharist is properly the sacrament of those who are in full communion with the Church.
1396 The unity of the Mystical Body: the Eucharist makes the Church. Those who receive the Eucharist are united more closely to Christ. Through it Christ unites them to all the faithful in one body - the Church. Communion renews, strengthens, and deepens this incorporation into the Church, already achieved by Baptism. In Baptism we have been called to form but one body. 233 The Eucharist fulfills this call: "The cup of blessing which we bless, is it not a participation in the blood of Christ? The bread which we break, is it not a participation in the body of Christ? Because there is one bread, we who are many are one body, for we all partake of the one bread:" 234
If you are the body and members of Christ, then it is your sacrament that is placed on the table of the Lord; it is your sacrament that you receive. To that which you are you respond "Amen" ("yes, it is true!") and by responding to it you assent to it. For you hear the words, "the Body of Christ" and respond "Amen." Be then a member of the Body of Christ that your Amen may be true. 235
1397 The Eucharist commits us to the poor. To receive in truth the Body and Blood of Christ given up for us, we must recognize Christ in the poorest, his brethren:
You have tasted the Blood of the Lord, yet you do not recognize your brother,. . . . You dishonor this table when you do not judge worthy of sharing your food someone judged worthy to take part in this meal. . . . God freed you from all your sins and invited you here, but you have not become more merciful. 236
1398 The Eucharist and the unity of Christians. Before the greatness of this mystery St. Augustine exclaims, "O sacrament of devotion! O sign of unity! O bond of charity!" 237 The more painful the experience of the divisions in the Church which break the common participation in the table of the Lord, the more urgent are our prayers to the Lord that the time of complete unity among all who believe in him may return.
1399 The Eastern churches that are not in full communion with the Catholic Church celebrate the Eucharist with great love. "These Churches, although separated from us, yet possess true sacraments, above all - by apostolic succession - the priesthood and the Eucharist, whereby they are still joined to us in closest intimacy." A certain communion in sacris, and so in the Eucharist, "given suitable circumstances and the approval of Church authority, is not merely possible but is encouraged." 238
1400 Ecclesial communities derived from the Reformation and separated from the Catholic Church, "have not preserved the proper reality of the Eucharistic mystery in its fullness, especially because of the absence of the sacrament of Holy Orders." 239 It is for this reason that, for the Catholic Church, Eucharistic intercommunion with these communities is not possible. However these ecclesial communities, "when they commemorate the Lord's death and resurrection in the Holy Supper . . . profess that it signifies life in communion with Christ and await his coming in glory." 240
1401 When, in the Ordinary's judgment, a grave necessity arises, Catholic ministers may give the sacraments of Eucharist, Penance, and Anointing of the Sick to other Christians not in full communion with the Catholic Church, who ask for them of their own will, provided they give evidence of holding the Catholic faith regarding these sacraments and possess the required dispositions. 241
226 Jn. 6:56.
227 Jn 6:57.
228 Fanqith, Syriac Office of Antioch, Vol. 1, Commun., 237 a-b.
229 PO 5.
230 St. Ambrose, De Sacr. 4, 6, 28: PL 16, 446; cf. I Cor 11:26.
231 Cf. Council of Trent (1551): DS 1638.
232 St. Fulgentius of Ruspe, Contra Fab. 28, 16-19: CCL 19A, 813-814.
233 Cf. I Cor 12:13.
234 I Cor 10:16-17.
235 St. Augustine, Sermo 272: PL 38, 1247.
236 St. John Chrysostom, Hom. in I Cor. 27, 4: PG 61, 229-230; cf. Mt 25:40.
237 St. Augustine, In Jo. ev. 26, 13: PL 35, 1613; cf. SC 47.
238 UR 15 § 2; cf. CIC, can. 844 § 3.
239 UR 22 § 3.
240 UR 22 § 3.
241 Cf. CIC, can. 844 § 4.
English Translation of the Cathechism of the Catholic Church for the United States of America © 1997, United States Catholic Conference, Inc.