Jamieson, Fausset and Brown on Matthew 12:39

39. But he answered and said unto them--"when the people were gathered thick together" (Lu 11:29).

an evil and adulterous generation--This latter expression is best explained by Jr 3:20, "Surely as a wife treacherously departeth from her husband, so have ye dealt treacherously with Me, O house of Israel, saith the Lord." For this was the relationship in which He stood to the covenant-people--"I am married unto you" (Jr 3:14).

seeketh after a sign--In the eye of Jesus this class were but the spokesmen of their generation, the exponents of the reigning spirit of unbelief.

and there shall no sign be given to it, but the sign of the prophet Jonas.