Matthew 13:10-17
10 Then the disciples came and said to him, “Why do you speak to them in parables?”
11 And he answered them, “To you it has been given to know the secrets of the kingdom of heaven, but to them it has not been given. 12 For to the one who has, more will be given, and he will have an abundance, but from the one who has not, even what he has will be taken away. 13 This is why I speak to them in parables, because seeing they do not see, and hearing they do not hear, nor do they understand. 14 Indeed, in their case the prophecy of Isaiah is fulfilled that says:
“‘You will indeed hear but never understand,
and you will indeed see but never perceive.
15 For this people’s heart has grown dull,
and with their ears they can barely hear,
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, and I would heal them.’
16 But blessed are your eyes, for they see, and your ears, for they hear. 17 For truly, I say to you, many prophets and righteous people longed to see what you see, and did not see it, and to hear what you hear, and did not hear it.
Also, in the parallel account in Mark:
Mark 4:10-12
10 And when he was alone, those around him with the twelve asked him about the parables. 11 And he said to them, “To you has been given the secret of the kingdom of God, but for those outside everything is in parables, 12 so that
“they may indeed see but not perceive,
and may indeed hear but not understand,
lest they should turn and be forgiven.”
And in Luke:
Luke 8:9-10
9 And when his disciples asked him what this parable meant, 10 he said, “To you it has been given to know the secrets of the kingdom of God, but for others they are in parables, so that ‘seeing they may not see, and hearing they may not understand.’
According to Jesus, the reason that He spoke in parables was so that God would leave certain people in their blindness, while at the same time, using them to reveal His truth to others (to whom these things had been “given”.)
“To you it has been given to know the secrets of the kingdom of heaven, but to them it has not been given.” – Matt 13:11