Luke 11:29-32 (ESV)
When the crowds were increasing, he began to say, "This generation is an evil generation. It seeks for a sign, but no sign will be given to it except the sign of Jonah.  For as Jonah became a sign to the people of Nineveh, so will the Son of Man be to this generation.  The queen of the South will rise up at the judgment with the men of this generation and condemn them, for she came from the ends of the earth to hear the wisdom of Solomon, and behold, something greater than Solomon is here.  The men of Nineveh will rise up at the judgment with this generation and condemn it, for they repented at the preaching of Jonah, and behold, something greater than Jonah is here.
The evil generation seeks a sign. Every generation that seeks a sign is evil. It betrays our evilness that we seek signs. Naturally we should just trust God’s word. We should just believe it. It is God who speaks. But we want signs. How often do we want signs? Ask yourself how often you have asked God for a sign before you were willing to act according to his will. We have a natural distrust of God. We don’t trust him. Like Gideon we keep laying out a fleece, asking the ground to be wet and the fleece dry, the fleece wet and the ground dry. In fact, I have even heard pastors tell their members to do such nonsense. God was patient with Gideon, but Gideon proved himself faithless and evil. This is the way it is for man who asks for signs rather than doing that which God commands. I am never really sure why it should be that an organization would name themselves after Gideon. The guy wins victory after victory for God, and then leads all of Israel into idolatry in celebration.
Jesus says there will be no sign but the sign of Jonah. It is a strange thing. Jonah serves as a type for Christ, spending 3 days in the belly of a fish, just as Jesus spends three days in the tomb. And this is often the sign that Jesus is said to be hinting at. Bo Giertz though draws attention to how Jonah converted Ninevah, a town that did not see any fish spit him out on the beach, with nothing but preaching.
The people had plenty of signs really. Jesus raised people from the dead, healed the sick, gave sight to the blind, but they were never enough. Yet it was those people who believed Jesus already that received the signs. Those who had no faith would receive no signs, and the signs they did receive were never enough to convince. Yet Sheba would come from the south to listen to Solomon, and Nineveh would repent at the mere preaching of Jonah. And so they will judge the evil generation that sought signs when someone greater than Solomon and Jonah were among them, who heard the word of God and believed because it was the word of God. Signs, and yet for those with eyes to see, they are all around us.