Luke 24:25
And he said to them, O dull of apprehension, and slow of heart to believe all that the Prophets have spoken!
Mark 7:18
And he saith to them, Are ye too so without discernment? Do ye not understand that whatever thing from without entereth into a man, cannot defile him?
Mark 8:17-18
And knowing it, he saith to them, Why are ye reasoning, because ye have no bread? Do ye not yet perceive nor understand? Have ye your mind still blinded?
Mark 9:19
He answereth and saith to them, O unbelieving generation! How long shall I be with you? How long shall I bear with you? Bring him to me.
Mark 16:14
Afterward he manifested himself to the eleven themselves, as they were reclining at table, and upbraided them with their unbelief and hardness of heart, because they did not believe those who had seen him after he had risen.
Hebrews 5:11-12
Of whom we have much to say, and hard to be explained, seeing ye have become dull of hearing.