‘And when the crowds were gathering together to him, he began to say, “This generation is an evil generation. It seeks after a sign. And there shall no sign be given to it but the sign of Jonah.” '
We are not told what initially prompted these words before the gathering crowds, probably a further request for a sign. But Luke does not mention it again for it has already been mentioned in Luke 11:16 (in a context where a comprehensive sign is given). Jesus was constantly being pressed to give signs by those who refused to believe the signs and wonders that they saw. But here He describes such requests as coming from an evil heart. If they cannot believe the signs that are revealed before them already then no sign will convince them, for these are the signs of the presence of the Kingly Rule of God. God will therefore at present give no further sign than He has already given them. For any further sign they must be content with ‘the sign of Jonah'. But what was ‘the sign of Jonah?'
When we hear the mention of Jonah's name there immediately springs to our minds his experience of being swallowed by a great fish. The same applied to Jesus' listeners. Otherwise why had He, from among all the prophets, selected Jonah?