Hebrews 5:11
Of whom we have much to say, and hard to be explained, seeing ye have become dull of hearing.
2 Peter 3:16
as also in all his letters, speaking in them of these things; in which things are some that are hard to be understood, which they that are unlearned and unstable wrest, as also the other Scriptures, to their own destruction.
Matthew 13:15
For this peoples heart hath become gross, and their ears are dull of hearing, and their eyes they have closed, lest they should see with their eyes, and hear with their ears, and understand with their heart, and turn from their ways, and I should heal them."
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 8:21
And he said to them, Do ye not yet understand?
Luke 24:25
And he said to them, O dull of apprehension, and slow of heart to believe all that the Prophets have spoken!
John 6:6
But this he said to try him; for he himself knew what he was going to do.
John 16:12
I have yet many things to say to you, but ye cannot bear them now.
Acts 28:27
For the heart of this people hath become gross, and their ears are dull of hearing, and their eyes they have closed; lest they should see with their eyes and hear with their ears, and understand with their heart and turn from their ways, and I should heal them."