John 16:18
So they asked one another repeatedly, "What can that 'little while' mean which He speaks of? We do not understand His words."
Matthew 16:9-11
Do you not yet understand? nor even remember the 5,000 and the five loaves, and how many basketfuls you carried away,
Luke 24:25
"O dull-witted men," He replied, "with minds so slow to believe all that the Prophets have spoken!
Hebrews 5:12
For although, considering the long time you have been believers, you ought now to be teachers of others, you really need some one to teach you over again the very rudiments of the truths of God, and you have come to require milk instead of solid food.