The scribes and Pharisees follow up, asking for a sign. Not all scribes are Pharisees and most Pharisees are not scribes, but these are probably Pharisees and some of their scribes.
Jesus’ answer is, basically, you have already seen enough. You don’t need any more signs.
But watch out for the biggie sign: it will remind you of Jonah in the belly of the fish for three days.
Then he goes on to talk about two times that gentiles turned to God even without such signs as the Pharisees have seen. First, the Ninevites responded to the message of Jonah even though they didn’t get to see the sign of Jonah and the message was delivered with reluctance.
Second, the Queen of Sheba turned to God because of the wisdom of Solomon.
These gentiles put the Pharisees to shame because they responded with lesser signs and to lesser messengers than Jesus.
Blessed are those who have not seen and yet still believe.