“An evil and adulterous generation seeks after a sign, and there will no sign be given to it, but the sign of Jonah.” And he left them, and departed.'
Then He points out what the nature is of those who seek spectacular signs in spiritual matters. They are ‘an evil and adulterous generation' (compare Matthew 12:39; Matthew 11:16). The seeking after signs, when such wonders have been done before them, is simply evidence of the evil of their hearts. ‘Adulterous' signifies a generation that is not in close touch with God, and is not truly seeking after God. Their minds are on other things, such as their own teaching and cleverness and self-importance. To such people no sign will be given, because they are unable to discern the true signs. Why, for those ready to see them, did not signs already abound? The problem lay not in Jesus' unwillingness to give signs, but in their inability to receive them. Those who will not respond to the signs that He has given have revealed themselves as not fit to be given any signs.
Thus the only sign that will be given to them is the sign of the prophet Jonah. Jonah came from the innards of a large fish to successfully evangelise Nineveh. One day they will see the Son of Man arise from the grave, and successfully evangelise the world (see on Matthew 12:38-42).
‘And he left them, and departed.' The statement indicates their rejection. In Matthew Jesus never ‘leaves' the crowds. It is only the opposition that He ‘leaves' in such a way. (He temporarily leaves the disciples when He goes away to pray, but there He does not ‘depart' - Matthew 26:44).
So the gradual increase of opposition now includes the Sadducees. He has been rejected by the Pharisees (Matthew 9:11; Matthew 9:34; Matthew 12:1-14; Matthew 12:24-32; Matthew 12:38; Matthew 16:1); by Scribes (Matthew 9:3; Matthew 12:38); by many of the common people (Matthew 11:16-19); by the towns of Galilee (Matthew 11:20-24;, by His own countrymen (Matthew 13:53-58); by Herod (Matthew 14:1-12); by the Pharisees and Scribes from Jerusalem (Matthew 15:1); and now by the Sadducees (Matthew 16:1). All that now awaits is His final rejection at Jerusalem.