The reason for speaking in parables (Mark 4:10-12; Luke 8:9-10). Because Christ's prejudiced hearers (see prefatory remarks) will not receive plain teaching, such as the Sermon on the Mount, they shall be punished by having the truth withdrawn from them, according to our Lord's own precept (Matthew 7:6), 'Grive not that which is holy unto the dogs, neither cast ye your pearls before swine, lest they trample them under their feet, and turn again and rend you.' But those hearers who are worthy, i.e. those who are of the household of faith, and already 'have' religious truth, shall understand.
Matthew 13:10-17
10 And the disciples came, and said unto him, Why speakest thou unto them in parables?
11 He answered and said unto them,Because it is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given.
12 For whosoever hath, to him shall be given, and he shall have more abundance: but whosoever hath not, from him shall be taken away even that he hath.
13 Therefore speak I to them in parables: because they seeing see not; and hearing they hear not, neither do they understand.
14 And in them is fulfilled the prophecy of Esaias, which saith, By hearing ye shall hear, and shall not understand; and seeing ye shall see, and shall not perceive:
15 For this people's heart is waxed gross, and their ears are dull of hearing, and their eyes they have closed; lest at any time they should see with their eyes, and hear with their ears, and should understand with their heart, and should be converted, and I should heal them.
16 But blessed are your eyes, for they see: and your ears, for they hear.
17 For verily I say unto you, That many prophets and righteous men have desired to see those things which ye see, and have not seen them; and to hear those things which ye hear, and have not heard them.