Luke 11:29
And when the people were gathered thick together, he began to say, This is an evil generation: they seek a sign; and there shall no sign be given it, but the sign of Jonas the prophet. And when the people were gathered thick together, he began to say, This is an evil generation: they seek a sign; and there shall no sign be given it, but the sign of Jonas the prophet.
The gathered crowd is rebuked as an evil generation for demanding an outward sign, because no such outward display could supply their deep need for spiritual renewal. No sign is to be given them except the sign of Jonah the prophet, which is Christ's time in the grave and His preaching to His own generation. Just as Jonah's preaching warned Nineveh and brought its people to repentance, so Christ's own preaching was itself the sign given to the generation that heard Him. The men of Nineveh will rise in the judgment and condemn those who rejected Him, for One greater than Jonah stood present among them.
Those who demand signs are not to look for wonders outside of God's Word. The sign to be manifested now and always is the Holy Spirit working upon the teacher's mind, so that the Word makes its fullest impression upon hearers. That Word is spirit and life and not dead theory, and to receive Christ unto eternal life is to feed upon Him through His Word.