Meeting Jesus in times of doubt
The fear and confusion of Good Friday are still very present for the disciples who have locked themselves away, and it is added to by the news from Peter and the disciple who Jesus loved, that they had been to the tomb and it was empty. Then they would have heard the testimony of Mary that she had been to the tomb and not only had she seen angels but she had seen and spoken to Jesus the risen Lord.
Our passage has two main themes, the first of which is faith. Thomas (often referred to as doubting Thomas), needs to see to believe. I think this is an understandable response on a number of levels. I often remind myself that doubt is not the opposite of faith, unbelief is. Doubt is a companion of faith; it can be a vehicle which takes you to a better understanding of what you really believe in.
The second main theme is mission. Jesus reminds this scared and then shocked group of followers that they are to continue a mission that started at creation. Jesus sends the disciples like the Father sent him, and he gives them the gift of the Holy Spirit, an assurance that God will always be present in their lives and that they will go out with his power to do his will.