Mark 8:21

And he said unto them, How is it that ye do not understand?

The question presses the disciples for still not grasping what the two feedings of the multitudes had shown about power to supply need, since after watching those miracles they still failed to trust for provision in the boat. Their dullness was not bare forgetfulness but unbelief sharpened by prejudice against the Gentiles who had just shared in the second feeding.

The same question also rebukes their failure to take in the saying about coming in glory with the glory of God and the heavenly hosts to give to every person according to works. It rebukes likewise their failure to take in the promise that some standing there would not taste death until they should see the Son of man coming in His kingdom. That coming glory looked distant to them, while their eyes stayed fixed on the nearer prospect of earthly poverty, humiliation, and suffering.

They held fast to bright expectations of the Messiah's kingdom and of seeing their Lord lifted to the throne of David. To minds shaped by that hope it was incomprehensible that the Son of God should go on as a humble homeless wanderer to be despised and rejected and put to death at Jerusalem, willingly giving Himself to such an end and leaving them in deeper darkness than before. Because they loved their Master their hearts sank with sadness, and at the same time their minds were tormented with doubt.