Here's the passage from The Message translation:
19-20Later on that day, the disciples had gathered together, but, fearful of the Jews, had locked all the doors in the house. Jesus entered, stood among them, and said, "Peace to you." Then he showed them his hands and side.
20-21The disciples, seeing the Master with their own eyes, were exuberant. Jesus repeated his greeting: "Peace to you. Just as the Father sent me, I send you."
22-23Then he took a deep breath and breathed into them. "Receive the Holy Spirit," he said. "If you forgive someone's sins, they're gone for good. If you don't forgive sins, what are you going to do with them?"
24-25But Thomas, sometimes called the Twin, one of the Twelve, was not with them when Jesus came. The other disciples told him, "We saw the Master."
But he said, "Unless I see the nail holes in his hands, put my finger in the nail holes, and stick my hand in his side, I won't believe it."
26Eight days later, his disciples were again in the room. This time Thomas was with them. Jesus came through the locked doors, stood among them, and said, "Peace to you."
27Then he focused his attention on Thomas. "Take your finger and examine my hands. Take your hand and stick it in my side. Don't be unbelieving. Believe."
28Thomas said, "My Master! My God!"
29Jesus said, "So, you believe because you've seen with your own eyes. Even better blessings are in store for those who believe without seeing."
30-31Jesus provided far more God-revealing signs than are written down in this book. These are written down so you will believe that Jesus is the Messiah, the Son of God, and in the act of believing, have real and eternal life in the way he personally revealed it.
I think the access we have to the lives of the apostles is very interesting. Other than the tiny bit we know about Phillip and Nathaniel, we generally hear the stories of the ones who either couldn't get Jesus' message or struggled with discipleship. The stories of Peter, James, John and Thomas give me hope for my own life of faith.
Here Thomas has gone out from the Upper Room. Though everyone was hiding in fear, he decides he needs to get out of there. Maybe he needed a little air, he went for food, he wanted to get a feel for how the town was reacting to the situation. Whatever he was doing, he missed Jesus coming back to the apostles. And (!) he missed the bestowal of the the Holy Spirit.
Here, the Holy Spirit is sent into the apostles, not for faith, but so they might be able to do the work of discipleship. You must forgive one another. Why do you think Jesus thought that was the most important issue? Don't you imagine they were all closed in that room and began to bicker about who could have prevented the crucifixion? Maybe they were steeped in anger at Judas. Jesus comes among them and gives the gift of the Spirit, so that they might forgive each other and know that they have been forgiven. They are not going to be able to fully comprehend the joy of the resurrection and Christ-among-them if they are not able to understand what it means not only for their relationship with God, but also for their relationship with one another.
Meanwhile, back at the ranch of doubt, Thomas comes back and it seems the whole group had a sincere of the risen Christ and HE MISSED IT!! Of course (!) his reaction is: "Yeah, right." But Jesus returns and Thomas is confronted with the reality of the risen Savior. What a moment!
Yet Jesus promises that the blessings of faith will be even greater for those who believe without seeing. That's us! Though we may understand ourselves to have encountered Christ, through other people, in sacramental life, we have not had the privilege of touching the wounds, of knowing what Thomas knows and sees.
Yet we are blessed. Faith is not the absence of doubt, it is action in spite of doubt. Though we struggle in the life of faith, Jesus has sent the Holy Spirit to us so that we may believe God's work continues through us, with us and for us. And, someday, we will be in the company of Thomas and others, in a place where we will be able to see and believe.