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.