Thomas needed to see the evidence of Jesus' death to believe that the living Jesus was the same man who died on the cross. In a similar way, Christians live in a world that needs to see the marks of our own death in Christ before they will believe what we preach about Jesus.
Jesus's resurrection proves the reality of the Christian hope for eternal life beyond death. It also demonstrates the divine power available to help Christians live Jesus' way now. Jesus calls Christians to store up treasures in heaven, because that is where our hearts will be. We also hear him saying that our hearts will have immense resources to serve God faithfully and to love others well. We need to be sure that we are investing out devotion, our time, and our money in that eternal hope.
John's story of "Doubting" Thomas teaches Christians the natural and spiritual response to an encounter with the risen Jesus. This challenges us as the Church to be the kind of Christians in whom others encounter Jesus and worship God.
Some Sadducees present Jesus with a "gotcha" question about belief in the resurrection of the dead. If God's word to Moses is true, and if the resurrection is true, then there is a problem. The Sadducees present a scenario in which these two things seem to contradict each other.
That is the way of skepticism. Skeptics and those determined to doubt everything can never be convinced beyond any doubt that they should become faithful.
Jesus shows that these kinds of apparent contradictions disappear when looking at the question in faith and hope. That is, knowing the power of God and believing his word, we can rest assured that God's promises answer all question.