Some of our Lord's opponents try to defend themselves by asking for a sign of his authority to claim so much; e.g. Matthew 12:30 (Matthew 12:38). In his reply he refers them to their own histories for proof that such a demand is inexcusable.
The Ninevites did not require one when Jonah became a sign to them—and in mentioning Jonah he refers to his being in the whale's belly three days and three nights as a symbol of what should happen to himself—and "the queen of the south" took immense trouble to satisfy her craving after wisdom (verss 39-42).
Therefore let them beware; their present state was one of extreme danger; the improvement that they showed was only negative, and if they did not take care worse would happen to them in the future than in the past (verses 43-45).