It is a common belief that in the scene in John 20 in which Jesus shows his wounds to Thomas, Jesus is chiding Thomas when he says, "Blessed are they who have not seen and yet believe." I've come to the conclusion that this is false. Thomas is an apostle, that is, an eyewitness. He has to see in order to be an Apostle. Our faith is based on the witness of the Apostles, so if they aren't witnesses, then how can we base our belief on their witness?
This passage is, in fact, establishing the bona fide of the apostolic witness by highlighting that Thomas and the other Apostles did see. Even Paul is an eyewitness of the resurrected Jesus, and hence an Apostle, though not one of the Twelve. And, of course, Matthias was selected because he was an eyewitness.
I wonder if anyone has done a study exploring why St. Thomas is so important in the Gospel of John. He appears several times. In the scene when Jesus starts towards Bethany to heal Lazarus, he says, "Let us also go, that we may die with him." At the Last Supper he asks, "Lord, we do not know where you are going; how can we know the way?" To which Jesus says "I am the way, and the truth, and the life; no one comes to the Father, but by me. If you had known me, you would have known my Father also; henceforth you know him and have seen him." And, of course, Thomas is the one who calls Jesus "My Lord and my God" directly. Finally, he appears on the shore of the sea with Peter, Nathaniel, the Sons of Zebedee, and two other disciples in the "coda" in Chapter 21.