Hebrews 5:11
Concerning whom there is much word to us, and difficult to speak, since you are dull of hearing.
2 Peter 3:16
as also in all his epistles, speaking in the same concerning these things; in which there are some things hard to understand, which the unlearned and unestablished wrest, as they also do the other scriptures, to their own destruction.
Matthew 13:15
Because the heart of this people is waxed gross, and they heard heavily with their ears, and closed their eyes; lest they may see with their eyes, and hear with their ears, and understand with their hearts, and turn, and I shall heal them.
Mark 8:17-18
And Jesus knowing says to them, Why do you reason because you have not bread? do you not yet know, nor understand? have you your heart still hardened?
Mark 8:21
And He said to them, Do you not yet understand?
Luke 24:25
And He said to them, O ye foolish, and slow in heart to believe in all those things which the prophet spoke!
John 6:6
He spoke this testing him; for He knew what He was about to do.
John 16:12
I have many things to say to you, but you are not able to bear them now.
Acts 28:27
for the heart of this people is waxed gross, and they heard with their ears heavily, and closed their eyes; lest they may see with their eyes, and hear with their ears, and understand with their heart, and turn, and I shall heal them.