Mark 8:12

he sighed deeply in his spirit Not merely, we may conclude, at their hardened disbelief, but also with the feeling that the decisive crisis of the severance from the ruling powers had come. "For the demand for a sign from heaven was a demand that He should, as the Messiah of their expectation, accredit Himself by a great overmastering miracle; thus it was fundamentally similar to the temptation in the wilderness, which He had repelled and overcome." Lange.

There shall no sign be given Literally, If a sign shall be given to this generation, a Hebrew form of strong abjuration. Comp. Hebrews 3:11, where see the margin; Mark 4:3; Mark 4:5; Genesis 14:23; Numbers 14:30. St Mark does not mention the sign of "Jonah the prophet" mentioned by St Matthew (Matthew 16:4).