From: John 20:19-23
Jesus Appears to the Disciples
19] On the evening of that day, the first day of the week, the doors shut where the disciples were, for fear of the Jews, Jesus came and stood among them and said to them, "Peace be with you."  When He had said this, He showed them His hands and His side. Then the disciples were glad when they saw the Lord.  Jesus said to them again, "Peace be with you. As the Father has sent Me, even so I send you."  And when He had said this, He breathed on them, and said to them, "Receive the Holy Spirit.  If you forgive the sins of any, they are forgiven; if you retain the sins of any, they are retained."
19-20. Jesus appears to the Apostles on the evening of the day of which He rose. He presents Himself in their midst without any need for the doors to be opened, by using the qualities of His glorified body; but in order to dispel any impression that He is only a spirit He shows them His hands and His side: there is no longer any doubt about its being Jesus Himself, about His being truly risen from the dead. He greets them twice using the words of greeting customary among the Jews, with the same tenderness as He previously used put into this salutation. These friendly words dispel the fear and shame the Apostles must have been feeling at behaving so disloyally during His passion: He has created the normal atmosphere of intimacy, and now He will endow them with transcendental powers.
21. Pope Leo XIII explained how Christ transferred His own mission to the Apostles: "What did He wish in regard to the Church founded, or about to be founded? This: to transmit to it the same mission and the same mandate which He had received from the Father, that they should be perpetuated. This He clearly resolved to do: this He actually did. `As the Father hath sent Me, even so I send you' (John 20:21). `As Thou didst send Me into the world, so I have sent them into the world' (John 17:18). [...] When about to ascend into Heaven, He sends His Apostles in virtue of the same power by which He had been sent from the Father; and He charges them to spread abroad and propagate His teachings (cf. Matthew 28:18), so that those obeying the Apostles might be saved, and those disobeying should perish (cf. Mark 16:16). [...] Hence He commands that the teaching of the Apostles should be religiously accepted and piously kept as if it were His own: `He who hears you hears Me, and he who rejects you rejects Me' (Luke 10:16). Wherefore the Apostles are ambassadors of Christ as He is the ambassador of the Father" ([Pope] Leo XIII, "Satis Cognitum"). In this mission the bishops are the successors of the Apostles: "Christ sent the Apostles as He Himself had been sent by the Father, and then through the Apostles made their successors, the bishops, sharers in His consecration and mission. The function of the bishops' ministry was handed over in a subordinate degree to priests so that they might be appointed in the order of the priesthood and be co-workers of the episcopal order for the proper fulfillment of the apostolic mission that had been entrusted to it by Christ" (Vatican II, "Presbyterorum Ordinis", 2).
22-23. The Church has always understood--and has in fact defined--that Jesus Christ here conferred on the Apostles authority to forgive sins, a power which is exercised in the Sacrament of Penance. "The Lord then especially instituted the Sacrament of Penance when, after being risen from the dead, He breathed upon His disciples and said: "Receive the Holy Spirit...' The consensus of all the Fathers has always acknowledged that by this action so sublime and words so clear the power of forgiving and retaining sins was given to the Apostles and their lawful successors for reconciling the faithful who have fallen after Baptism" (Council of Trent, "De Paenitentia", Chapter 1).
The Sacrament of Penance is the most sublime __expression of God's love and mercy towards men, described so vividly in Jesus' parable of the prodigal son (cf. Luke 15:11-32). The Lord always awaits us, with His arms wide open, waiting for us to repent--and then He will forgive us and restore us to the dignity of being His sons.
The Popes have consistently recommended Christians to have regular recourse to this Sacrament: "For a constant and speedy advancement in the path of virtue we highly recommend the pious practice of frequent Confession, introduced by the Church under the guidance of the Holy Spirit; for by this means we grow in a true knowledge of ourselves and in Christian humility, bad habits are uprooted, spiritual negligence and apathy are prevented, the conscience is purified and the will strengthened, salutary spiritual direction is obtained, and grace is increased by the efficacy of the Sacrament itself" ([Pope] Pius XII, "Mystici Corporis").
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.