(from the http://www.bibleinfo.com/en/questions/who-were-twelve-disciples webpage)
Thomas Didymus lived in Galilee. Tradition says he labored in Parthia, Persia, and India, suffering martyrdom near Madras, at Mt. St. Thomas, India.
Thomas was his Hebrew name and Didymus was his Greek name. At times he was called Judas. Matthew, Mark and Luke tell us nothing about Thomas except his name. However, John defines him more clearly in his Gospel. Thomas appeared in the raising of Lazarus (John 11:2-16), in the Upper Room (John 14:1-6) where he wanted to know how to know the way where Jesus was going. In John 20:25, we see him saying unless he sees the nailprints in Jesus' hand and the gash of the spear in His side he will not believe. That's why Thomas became known as Doubting Thomas.
Thomas became certain by doubting. By nature, he was a pessimist. He was a bewildered man. Yet, he was a man of courage. He was a man who could not believe until he had seen. He was a man of devotion and of faith. When Jesus rose, he came back and invited Thomas to put his finger in the nail prints in his hands and in his side. Here, we see Thomas making the greatest confession of faith, "My Lord and my God." Thomas' doubts were transformed into faith. Thomas was always like a little child. His first reaction was not to do what he was told to do and not to believe what he was asked to believe. The good news to him was always too good to be true. By this very fact Thomas' faith became great, intense and convincing. It is said that he was commissioned to build a palace for the king of India, and he was killed with a spear as a martyr for his Lord. His symbol is a group of spears, stones and arrows.