Why did the Pharisees demand a sign, if Jesus has been doing the messianic signs all along?
Signs can be misinterpreted.
If Jesus commands demons, does that authority come from God or from the Satan? Mark 8:11 and Luke 11:16 says the request is a “sign from heaven,” Matthew omits “from heaven.” The Pharisees what to see something that certainly comes from God.
There is malicious intent behind this request. Both Mark 8:11-12 and Luke 11:16 say the Pharisees were trying to test (πειράζω) him. Although Matthew does not directly state this is a test, the Pharisees are seeking a way to accuse Jesus. If Jesus does a sign on demand, perhaps they can twist that into something they can use. In Matthew 12:9-10 the Pharisees get Jesus to heal on the Sabbath because they were looking for a reason to bring charges against him.
Do the Pharisees mean to imply the signs and wonders Jesus has done are not messianic signs from God? The meaning of “sign” (σημεῖον) is important. This is more than a miracle, but this word refers to a wondrous deed that confirms the one doing the sign is from God.
What is a Sign?
A sign is “an event that is an indication or confirmation of intervention by transcendent powers” (BDAG). For example, Jesus did many miracles, but John records seven signs which prove Jesus is the messiah, and that by believing readers can have life in his name (John 20:30-31). In Isaiah 7, when the Lord promised Ahaz he would be saved from this political enemies, he tells the king to ask for a sign. The king refused, but was given the sign of Immanuel anyway. When King Hezekiah was healed he asks “what will be the sign this is so?”
In Acts miracles accompany the preaching of the Gospel so that the audience knows the speaker is a representative of the messiah. When Peter heals a lame man in Acts 3, he is doing the same kind of messianic sign Jesus did and the audience understands this sign as coming from God. Hebrews 2:3-4 says the salvation first announced by the Lord was testified by “signs, wonders and various miracles.”
Signs are not always indications a person is from God, false prophets can produce lying signs and wonders. Deuteronomy 13:1-5 indicates prophets may arise in Israel and do signs and wonders, yet they encourage the people to peruse other gods, they are to be rejected even if they have done signs and wonders. Jesus warns his disciples about false prophets doing sings and wonders (Matt 24:24). Finally, Paul warns the coming Man of Sin will do false signs and wonders (2 Thess 2:9).
Jews Demand a Sign
Paul famously said the Jews demand signs; Greeks seek wisdom, “but we preach Christ crucified” (1 Cor 1:22). When Peter healed the lame man, the Jews understood the miracle as a messianic sign; when Paul healed a lame man in Acts 14 the Gentiles in Lystra misunderstood the sign as a miracle performed by the gods Zeus and Hermes (and chaos ensues).
In the years leading up to the Jewish revolt, there were a number of “Jewish Sign Prophets” (to use Barnett’s phrase) who claimed to do some sort of sign in anticipation of God sending his messiah or messianic kingdom.
Most styled themselves as a new Moses, leading people out into the wilderness to renew Israel. Some may have done signs, most claimed they were do a great sign (destroy the walls of Jerusalem, for example). They all developed a little following and eventually came to nothing (usually killed by the Romans). These are the men Gamaliel mentions in Acts 5:33-39.
But Barnett points out Josephus never describes them as messiahs. He never used the term Χριστός even if they considered themselves to be kings or messianic figures. The Egyptian, for example, styled himself as a king. Barnett does not think these prophets were charlatans who were pushing a political agenda by faking signs. The signs promised by these messianic prophets intended to “force the hand of God to speedily bring his ‘Salvation’” (p. 688). They “sought to activate God’s eschatological salvation” (p. 693).
Barnett suggests these prophets did some sort of Exodus-Conquest signs (crossing the Jordan, knocking down the walls of Jerusalem like Jericho). If this was the case, perhaps the Pharisees expected Jesus so follow up his implicit claim to be a new Moses with some miracle drawn from the Exodus or the Conquest. In Matthew 14, Jesus will do just that, leading his followers into the wilderness he will give them food, recalling the miraculous feeding of Israel in the wilderness.
The Jews expected the messiah to do signs and wonders, which is why Jesus did many signs and wonders. But in this case, the sign he will give points back to Jonah, while looking forward to his resurrection.
Bibliography: P. W. Barnett, “The Jewish Sign Prophets – A.D. 40-70: Their Intention and Origin.” NTS 27 (1980-81):679-97.