The Bible was written to separate "the wheat from the chaff ", those who are really desirous of knowing "the truth" as opposed to those who are unwilling to "dig deep" for the nuggets of "truth" that are found in the Bible, with most being satisfied with the "status quo" that the churches present.
After performing "powerful works" in Chorazin, Bethsaida and Capernaum (his home territory), and these being unresponsive, Jesus then said in prayer to his Father: "I publicly praise you, Father, Lord of heaven and earth, because you have hidden these things from the wise and intellectual ones and have revealed them to babes. Yes, O Father, because to do thus came to be the way approved by you."(Matt 11:25, 26)
Jesus was fully aware that his Father has written the Bible (at that time, the Hebrew Scriptures, commonly called the Old Testament) in a way as to exclude "wise and intellectual ones" from accurately understanding it (which included the reason for the powerful works in these 3 cities), but only to "babes", those who are humble enough to be taught by Jehovah (John 6:45; Isa 54:13), to allow the Bible speak for itself rather than forcing "a square peg in a round hole", as the churches have done.
Thus, at Matthew 13, Jesus used the Greek word syniemi some 6 times that means "to mentally put the pieces together."(Matt 13:13-15, 19, 23, 51) It is translated as "understand" in the King James Bible, but is more accurately is rendered as "getting the sense of it". The word "understand" is more readily rendered from the Greek word ginosko, meaning "to know, understand, perceive, have knowledge of."(Thayer's Greek-English Lexicon of the New Testament, pg 117)
At Matthew 13:11, Jesus told his genuine disciples: "To you it is granted to understand (Greek gnonai ) the sacred secrets of the kingdom of the heavens, but to those people it is not granted." And then he explains why, saying: "This is why I speak to them by the use of illustrations, because, looking, they look in vain, and hearing, they hear in vain, neither do they get the sense of it." ("get the sense of it", Greek syniemi)
By using illustrations that would be very difficult to grasp, Jesus intentionally put a separation between those who would be his genuine disciples and those who are pseudo-disciples or false disciples. The fake disciples would fail to exert themselves "diligently" in order to understand the meaning of the illustrations that Jesus gave.(Luke 13:24) It would be too much of a "brain teaser", so that they "look in vain" and "hear in vain".