Hebrews 5:11
Of him we have many things to say, and difficult to be explained, if we do say them, since you are slow in understanding.
2 Peter 3:16
as also in all his letters, speaking in them of these things, in which are some things hard to be understood, which the unlearned and unstable wrest, as they do also the other scriptures, to their own destruction.
Matthew 13:15
For the heart of this people has become fat; and with their ears they hear heavily; and their eyes they have closed; lest they should see with their eyes, and hear with their ears, and understand with their heart, and should turn to me, and I should give them health.
Mark 8:17-18
And Jesus perceived it, and said to them: Why do you reason, because you have no bread? Do you not yet perceive, nor understand? Is your heart still hardened?
Mark 8:21
And he said to them: How is it that you do not understand?
Luke 24:25
And he said to them: inconsiderate, and slow of heart to believe all things that the prophets have spoken!
John 6:6
But this he said to try him; for he himself knew what he was about to do.
John 16:12
I have yet many things to say to you; but you can not bear them now.
Acts 28:27
For the heart of this people has become fat, and with their ears they hear heavily, and their eyes they have closed, lest they should see with their eyes, and hear with their ears, and understand with their heart, and should turn, and I should restore them to health.