We give Thomas a hard time. It wasn't his fault that he was out buying groceries, or whatever reason he wasn't trapped by fear with the others in that room. Maybe he just wasn't as afraid.
Thomas, from the little we know of him, was the practical one. He was the one, when Jesus said they were going to Bethel to see dead Lazarus, "Let's all go and die with him." It might very well be that Thomas was just getting on with his life. It was really too bad that Jesus died, horrible really, but he was gone and and the cupboard is empty....
So Thomas wasn't there that first Easter Sunday evening when Jesus appears to the rest of the gang. He shows them his wounds, eats fish and generally set their minds at ease. This is real. When they tell Thomas, he wants the same level of proof that the others got. It's only fair. Jesus seems to think so. The next Sunday he shows up again and gives Thomas exactly what he asked for.
There is a fundamental shift in Thomas' perspective. He drops to his knees and cries out "My Lord and My God." I think this is the moment of his real call. Up to this second he was on board because it was the logical thing to do. He wanted to change the world and Jesus looked like a good bet. It would explain his lack of fear the week before. It wasn't that he didn't care, but it wasn't personal.
When Jesus shows up with his wounds and his knowledge of Thomas' doubt, the world shifts and suddenly Thomas is following is Jesus the person, not Jesus the idea. It is at this point that a lot of us get hung up. We are dedicated to Jesus the idea. Either the idea is not a very powerful one and it doesn't move us past our intellectual coolness, or the idea is a very emotional one and we need to protect it fiercely against all threats so that it doesn't get lost.
In either case doubt is a problem. If we are just intellectual Christians, than doubt is immaterial. We can change and shift our idea so it still matches what we want from it. We can even give it up and follow a different idea because it doesn't connect with the rest of our being. It never becomes more than an idea.
With the emotional connection to the idea, doubt is a threat. If we doubt, question, probe the idea maybe it will lose its power and our connection to it will fail. So the idea never really gets tested.
Either way we are a people who hate doubt. We want to know with exactitude. We have polls and surveys. We bring up statistics. We create scientific models. We argue, but we rarely doubt, we rarely question ourselves in the midst of screaming questions at other.
Doubt is a powerful pathway to faith. Thomas doubted and the resolution of his doubt changed him. He brought Christianity to India. He was driven, not by pragmatism, but by his relationship with the person Jesus.
Doubt pushes us to go deeper, past the intellectual, past the emotions to a place where we don't find certainty, but a person.