You are strongly recommended to add to your library the excellent revised edition of Harper's Bible Dictionary titled, The Harper Collins Bible Dictionary, Revised Edition, edited by Paul J. Achtemeier, with the Society of Biblical Literature (NY: Harper Collins, 1996). It is currently the best one-volume Bible dictionary in English, and it is available at Border's Books, Christian Science Reading Rooms, http://www.borders.com, or http://www.christianbook.com.

Thomas, one of the twelve disciples* or apostles of Jesus, called ‘Didymus’ (‘twin’) in the Gospel of John (John 11:16; 20:24; 21:2). He appears in each of the apostolic lists (Matt. 10:3; Mark 3:18; Luke 6:15; Acts 1:13). Receiving little mention in the synoptic Gospels, Thomas becomes important in the later portions of the Fourth Gospel. He alone appears to be a tower of strength when he encourages the disciples to accompany Jesus into a hostile Judea even if it means death (John 11:16). He appears to be without understanding when, in John 14:5, he confesses his ignorance about where Jesus is going and therefore finds it difficult to follow him. He is most commonly remembered as the ‘doubting Thomas’ who refused to believe in Jesus’ resurrection until he saw the scars and was invited to place his fingers where the nails were driven and his hand into Jesus’ side (John 20:24-29). The story stands as a paradigm for all Christians who are called to believe in Christ without having seen him or having been granted tangible proof of his existence (v. 29). Thomas’s response is that of all who later believe: ‘My Lord and my God!’ (v. 28). In John 21:1-14, Thomas is one of the small group of disciples who go fishing and then see the risen Lord.

Little is known about Thomas’s activities after the crucifixion of Jesus. He is recorded as among those gathered in the upper room after the ascension (Acts 1:13). Thereafter, tradition preserves only legendary stories of little apparent historical value. A Gnostic apocryphal gospel known as the Gospel of Thomas is attributed to this Thomas.

Beyond Belief: The Secret Gospel of Thomas

by Elaine Pagels (NY: Random House, 2003, page 58, 69-73. For more details, see the entire book.)

As the scholar Gregory Riley points out, John -- and only John -- presents a challenging and critical portrait of the disciple he calls "Thomas, the one called Didymus," and, as Riley suggests, it is John who invented the character we call Doubting Thomas, perhaps as a way of caricaturing those who revered a teacher -- and a version of Jesus' teaching -- that he regarded as faithless and false. The writer called John may have met Thomas Christians among people he knew in his own city -- and may have worried that their teaching would spread to Christian groups elsewhere. John probably knew that certain Jewish groups -- as well as many pagans who read and admired Genesis 1 -- also taught [as did Thomas] that the "image of God" was within humankind [see also Luk 17:20,21]; in any case, John decided to write his own gospel insisting that it is Jesus -- and only Jesus -- who embodies God's word, and therefore speaks with divine authority... (page 58)

Mark, Matthew, and Luke mention Thomas only as one of "the twelve." John singles him out as "the doubter" -- the one who failed to understand who Jesus is, or what he is saying, and rejected the testimony of the other disciples. John then tells how the risen Jesus personally appeared to Thomas in order to rebuke him, and brought him to his knees. From this we might conclude, as most Christians have for nearly two millennia, that Thomas was a particularly obtuse and faithless disciple -- though many of John's Christian contemporaries revered Thomas as an extraordinary apostle, entrusted with Jesus' "secret words." ... (pages 70-71)

Luke specifies that, after the crucifixion, the risen Jesus appeared to "the eleven," and Matthew agrees that he appeared to "the eleven disciples" -- all but Judas Iscariot -- and conferred the power of the holy spirit upon "the eleven." But John's account differs. John says instead that "Thomas, called 'the twin' ... was not with them when Jesus came."

According to John, the meeting Thomas missed was crucial; for after Jesus greeted the ten disciples with a blessing, he formally designated them his apostles: "As the Father has sent me, so I send you." Then he "breathed upon them" to convey the power of the holy spirit; and finally he delegated to them his authority to forgive sins, or to retain them. The implication of the story is clear: Thomas, having missed this meeting, is not an apostle, has not received the holy spirit, and lacks the power to forgive sins, which the others received directly from the risen Christ. (page 71)

Addressing those who see Jesus differently [than John teaches], John urges his uncompromising conviction: belief in Jesus alone offers salvation. To those who heed, John promises great reward: forgiveness of sins, solidarity with God's people, and the power to overcome death. In place of Thomas's cryptic sayings, John offers a simple formula, revealed through the story of Jesus' life, death, and resurrection: "God loves you; believe, and be saved." ...

John, of course, prevailed... (page 72)

But the discovery of Thomas's gospel shows us that other early Christians held quite different understandings of "the gospel." For what John rejects as religiously inadequate -- the conviction that the divine dwells as "light" within all beings -- is much like the hidden "good news" that Thomas's gospel proclaims. Many Christians today who read the Gospel of Thomas assume at first that it is simply wrong, and deservedly called heretical. Yet what Christians have disparagingly called gnostic and heretical sometimes turn out to be forms of Christian teaching that are merely unfamiliar to us -- unfamiliar precisely because of the active and successful opposition of Christians such as John. (page 73)

See more information, tradition, and legend about Thomas and his travels after Jesus' resurrection at :

http://www.bibletexts.com/glossary/thetwelve.htm#thomas.