From: John 20:24-29
Jesus Appears to the Disciples (Continuation)
 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."
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).
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.