‘But he answered and said to them, “An evil and adulterous generation seeks after a sign, and there will no sign be given to it apart from the sign of Jonah the prophet,” '
‘Answered and said.' In Matthew ‘answered' does not necessarily refer back to a particular question. It rather has in mind that Jesus' words are a total answer to all who hear them, that Jesus is God's answer to all men's questions.
Jesus' reply is that the only reason that the present generation, who had seen His works (Matthew 11:5; Matthew 11:20-23), could possibly want a sign was because they were ‘evil and adulterous', that is, because they had made ritual their god and their hearts were set on anything but God, with the result that they were unable to truly judge and weigh up His moral teaching and His life, and recognise that both were from God. For had they but recognised it, His unique teaching was the greatest sign of all, a sign which has carried on through the ages, leaving all who have read it without excuse. Our present generation also will be judged by the fact that it has the teaching of Jesus in all its splendour, and yet have refused to acknowledge it in any significant way, and to respond to the One Who gave it. The principle, ‘If any man wills to do His will, He will know of the teaching, whether it be of God' (John 7:17), still applies. Thus all men and women will be judged by their reaction to that teaching and to the One Who is its source.
They thus have all the proof that they need before their eyes. Nineveh had recognised such teaching, even from a lesser man, and so had the queen of the south, Gentiles all, and all had responded to it, but the Jews who now had something greater, had not because they were evil and adulterous. They had replaced God in their hearts by His own Law as interpreted by them, and had gone astray after ritual, while at the same time altering His Law to suit their own ideas. For such spiritual adultery see also Matthew 16:4: Mark 8:38; Isaiah 57:3; Ezekiel 16:32; Ezekiel 16:36; Ezekiel 23:37; Ezekiel 23:45; Hosea 1-3.
Furthermore it also signified that they were unbelieving and faithless, and that they had forsaken the true God in their hearts, judging Him only in unworthy ways, and refusing to believe unless He did things their way, and continually proved Himself. They wanted constant evidences, thereby underlining their unbelief. It is one of the ironies of history that the very people who always wanted to see the spectacular, had on the other hand taken God's living Law and turned it into chains and manacles as a result of its formality and excessive restraint. They were like the adulterous wife of Hosea (Hosea 1:2), taken up with anyone and anything except faithfulness, except what represented the truth. Josephus later tells us that they were also evil and adulterous in practise, (‘that period had somehow become so prolific of crime of every description amongst the Jews, that no deed of iniquity was left unperpetrated, nor, had man's wit been exercised to devise it, could he have discovered any new form of vice' (not already being practised)) which was the kind of behaviour that went along with their spiritual attitude.
For their being ‘evil' compare Matthew 12:34. There they had revealed their evil by how they had responded to His casting out of demons. Here it was by how they responded to Him and to His teaching.
So Jesus warns them that He will only give them one sign, the sign of the prophet Jonah, although, as Mark 8:11-12 brings out, it is not the kind of sign that they are looking for. That kind of sign He will not give them! Jonah was called by God to preach in godless Nineveh, but he tried to avoid the task and ended up within the insides of a large fish. Released from there after three days, through fervent prayer, he went and preached in Nineveh where there was instant revival, with the people repenting in sackcloth and ashes (a sign of their repentance). Jesus will use both these ideas for His signs.