From the Treasury of Daily Prayer:

All four Gospels mention St. Thomas as one of the twelve disciples of Jesus. John’s Gospel, which names him “the Twin,” uses Thomas’s questions to reveal truths about Jesus. It is Thomas who says, “Lord, we do not know where You are going. How can we know the way?” To this question Jesus replies, “I am the way, and the truth, and the life” (John 14:5-6). John’s Gospel also tells how Thomas, on the evening of the day of Jesus’ resurrection, doubts the report of the disciples that they had seen Jesus. Later, “doubting Thomas” becomes “believing Thomas” when he confesses Jesus as “my Lord and my God” (John 20:24-29). According to tradition, Thomas traveled eastward after Pentecost, eventually reaching India, where still today a group of people call themselves “Christians of St. Thomas.” Thomas was martyred for the faith by being speared to death.