From: John 20:19-31
Jesus Appears to the Disciples
 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."
 Now Thomas, one of the Twelve, called the Twin, was not with them when Jesus came.  So the other disciples told him, "We have seen the Lord." But he said to them, "Unless I see in His hands the print of the nails, and place my finger in the mark of the nails, and place my hand in His side, I will not believe."
 Eight days later, His disciples were again in the house, and Thomas was with them. The doors were shut, but Jesus came and stood among them, and said, "Peace be with you."  Then He said to Thomas, "Put your finger here, and see My hands; and put out your hand, and place it in My side; do not be faithless, but believing."  Thomas answered Him, "My Lord and my God!"  Jesus said to him, "Have you believed because you have seen Me? Blessed are those who have not seen and yet believe."
 Now Jesus did many other signs in the presence of the disciples, which are not written in this book;  but these are written that you may believe that Jesus is the Christ, the Son of God, and that believing you may have life in His name.
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").
24-28. Thomas' doubting moves our Lord to give him special proof that His risen body is quite real. By so doing He bolsters the faith of those who would later on find faith in Him. "Surely you do not think", [Pope] St. Gregory the Great comments, "that is was a pure accident that the chosen disciple was missing; who on his return was told about the appearance and on hearing about it doubted; doubting, so that he might touch and believe by touching? It was not an accident; God arranged that it should happen. His clemency acted in this wonderful way so that through the doubting disciple touching the wounds in His Master's body, our own wounds of incredulity might be healed. [...] And so the disciple, doubting and touching, was changed into a witness of the truth of the Resurrection" ("In Evangelia Homiliae", 26, 7).
Thomas' reply is not simply an exclamation: it is an assertion, an admirable act of faith in the divinity of Christ: "My Lord and my God!" These words are an ejaculatory prayer often used by Christians, especially as an act of faith in the real presence of Christ in the Blessed Eucharist.
29. [Pope] St. Gregory the Great explains these words of our Lord as follows: "By St. Paul saying `faith is the assurance of things hoped for, the conviction of things unseen' (Hebrews 11:1), it becomes clear that faith has to do with things which are not seen, for those which are seen are no longer the object of faith, but rather of experience. Well then, why is Thomas told, when he saw and touched, `Because you have seen, you have believed?' Because he saw one thing, and believed another. It is certain that mortal man cannot see divinity; therefore, he saw the man and recognized Him as God, saying, `My Lord and my God.' In conclusion: seeing, he believed, because contemplating that real man he exclaimed that He was God, whom he could not see" ("In Evangelia Homiliae", 27, 8).
Like everyone else Thomas needed the grace of God to believe, but in addition to this grace he was given an exceptional proof; his faith would have had more merit had he accepted the testimony of the other Apostles. Revealed truths are normally transmitted by word, by the testimony of other people who, sent by Christ and aided by the Holy Spirit, preach the deposit of faith (cf. Mark 16:15-16). "So faith comes from what is heard, and what is heard comes from the preaching of Christ" (Romans 10:17). The preaching of the Gospel, therefore, carries with it sufficient guarantees of credibility, and by accepting that preaching man "offers the full submission of his intellect and will to God who reveals, willingly assenting to the revelation given" (Vatican II, "Dei Verbum", 5).
"What follows pleases us greatly: `Blessed are those who have not seen and yet believe.' For undoubtedly it is we who are meant, who confess with our soul Him whom we have not seen in the flesh. It refers to us, provided we live in accordance with the faith, for only he truly believes who practices what the believes" ("In Evangelia Homiliae", 26, 9).
30-31. This is a kind of first epilogue or conclusion to the Gospel of St. John. The more common opinion is that he added Chapter 21 later, which covers such important events as the triple confession of St. Peter, confirmation of his primacy and our Lord's prophecy about the death of the beloved disciple. These verses sum up the inspired writer's whole purpose in writing his Gospel--to have men believe that Jesus was the Messiah, the Christ announced by the prophets in the Old Testament, the Son of God, so that by believing this saving truth, which is the core of Revelation, they might already begin to partake of eternal life (cf. John 1:12, 2:23; 3:18; 14:13; 15:16; 16:23-26).
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.