Jesus, unlike many entertainment corporations today, does not offer a show upon demand. When the Pharisees ask for a sign it further reveals their attitude toward the miracle Jesus has just performed (Matthew 12:22, 25). It reveals their attitude toward all of His miracles, their attitude toward Jesus himself. The Pharisees do not need a new sign, they need new hearts, for they cannot see the kingdom of God (John 3:3).
And yet Jesus promises them a sign. What does Jesus mean that He will give them no sign except the resurrection? He has give them many signs, and He will perform many more. If we look at some instances of the apostolic preaching of the resurrection where this “evil and adulterous generation” is also mentioned I think we will see what Jesus means (all emphases mine).
[T]his Jesus, delivered up according to the definite plan and foreknowledge of God, you crucified and killed by the hands of lawless men. God raised him up, loosing the pangs of death, because it was not possible for him to be held by it. – Acts 2:23-24
The God of Abraham, the God of Isaac, and the God of Jacob, the God of our fathers, glorified his servant Jesus, whom you delivered over and denied in the presence of Pilate, when he had decided to release him. But you denied the Holy and Righteous One, and asked for a murderer to be granted to you, and you killed the Author of life, whom God raised from the dead. To this we are witnesses. – Acts 3:13-15
For those who live in Jerusalem and their rulers, because they did not recognize him nor understand the utterances of the prophets, which are read every Sabbath, fulfilled them by condemning him. And though they found in him no guilt worthy of death, they asked Pilate to have him executed. And when they had carried out all that was written of him, they took him down from the tree and laid him in a tomb. But God raised him from the dead,
[B]ecause he [God] has fixed a day on which he will judge the world in righteousness by a man whom he has appointed; and of this he has given assurance to all by raising him from the dead.” – Acts 17:31
[Jesus] and was declared to be the Son of God in power according to the Spirit of holiness by his resurrection from the dead, Jesus Christ our Lord, – Romans 1:4
The resurrection is the vindication of Jesus. It shows the wicked and adulterous generation to be wrong in their opinion concerning Jesus. They kill Him, God raises Him from the dead. And still today the resurrection is the sign that all doubters, skeptics, agnostics, and atheists have to deal with (toward understanding and using the apologetic strength of the resurrection I recommend beginning with The Reason for God, specifically chapter 13).
So to all who ask, “Why on earth doesn’t God do something? Why doesn’t He prove Himself?” We reply that God has indeed on earth done something; in the incarnation and resurrection of Jesus Christ and everything in between, God has indeed on earth done something. To those who ask a sign to validate our message I love the reply of Calvin. When Roman Catholics contended that their miracles authenticate their message, and then asked where the reformers miracles were, Calvin replied:
In demanding miracles of us, they act dishonestly. For we are not forging some new gospel, but are retaining that very gospel whose truth all the miracles that Jesus Christ and His Disciples ever wrought serve to confirm.
Doubters, skeptics, agnostics, and atheists don’t need new signs, they need their eyes open to the signs that are.