Mark 6:52
for they had not understood the lesson of the loaves; their minds were dull.
Mark 3:5
Then glancing round him in anger and vexation at their obstinacy he told the man, "Stretch out your hand." He stretched it out and his hand was quite restored.
Mark 8:17-21
He noted this and said to them, "Why do you argue you have no bread? Do you not see, do you not understand, even yet? Are you still dull of heart?
Matthew 16:9-11
Do you not understand even yet? Do you not remember the five loaves of the five thousand and how many baskets you took up?
Mark 7:18
He said to them, "So you do not understand, either? Do you not see how nothing outside a man can defile him by entering him?
Mark 16:14
Afterwards he appeared at table to the eleven themselves and reproached them for their unbelief and dulness of mind, because they had not believed those who saw him risen from the dead. [But they excused themselves, saying, "This age of lawlessness and unbelief lies under the sway of Satan, who will not allow what lies under the unclean spirits to understand the truth and power of God; therefore," they said to Christ, "reveal your righteousness now." Christ answered them, "The term of years for Satan's power has now expired, but other terrors are at hand. I was delivered to death on behalf of sinners, that they might return to the truth and sin no more, that they might inherit that glory of righteousness which is spiritual and imperishable in heaven."]
Luke 24:25
He said to them, "O foolish men, with hearts so slow to believe, after all the prophets have declared!
Romans 11:7
Now what are we to infer from this? That Israel has failed to secure the object of its quest; the elect have secured it, and the rest have been rendered insensible to it ??8 as it is written, God has given them a spirit of torpor, eyes that see not, ears that hear not ??down to this very day.