Thomas answered and said to him, "My Lord and my God!" Jesus said to him, "Because you have seen Me, have you believed? Blessed are they who did not see, and yet believed." Therefore many other signs Jesus performed in the presence of the disciples, which are not written in this book; but these have been written that you may believe that Jesus is the Christ, the Son of God; and that believing you may have life in His name. John 20:28-31, NASB.
His doubts removed, all Thomas could say is "My Lord and my God!" The hesitant disciple had come to faith. And that faith would shape the rest of his life. While the book of Acts (which focuses on Peter's proclamation in Jerusalem in the early part and Paul's work for the Gentiles in the latter) does not feature his future activities, a strong tradition in the early church tells us that Thomas, the courageous disciple who needed to know for sure before he moved, became the pioneer missionary to Persia and India. Southern India has an indigenous group known for centuries as Thomas Christians. They claim that he suffered martyrdom on St. Thomas Mount near Madras.
While we cannot prove that tradition (at least to the exacting standards of Thomas himself), we do know beyond a shadow of a doubt what drove the man for the rest of his life--the conviction that Jesus is "my Lord and my God." That firm belief became the guiding force and anchor point in his life.
But more important yet, it is the foundation for all Christianity. With that declaration by Thomas the Gospel of John has come full circle. Thomas is the first person in John to address Jesus as "God." Yet that is the truth that begins the fourth Gospel: "In the beginning was the Word, and the Word was with God, and the Word was God....And the Word became flesh and dwelt among us" (John 1:1-14, RSV).
After that introduction, John guides his readers in a journey of discovery as we ourselves come to the conclusion, through the experiences of Thomas and others, that Jesus of Nazareth is indeed the Messiah, the Son of God. John tells us in his statement of purpose that he selected the material for his Gospel so that we might come to the same conclusion as Thomas ("That you may believe that Jesus is the Christ, the Son of God") and thereby obtain life eternal (John 20:30, 31, RSV).
John sought to demonstrate that apostolic conviction as firmly and certainly as possible because he knew that we could never be first-person witnesses to the Resurrection, but would of necessity have to rely on inspired Scripture. Thus "blessed are those who have not seen and yet believe" (Verse 29, RSV).