Jn 12:36-41, Mt 13:13-17, Mk 4:10-12, Lk 8:10, Acts 28:23-28, Rom 11:7-8

In the Old Testament, Isaiah said that God had kept the Israelites from understanding God's Word. (Isa 6:9-10)

Isa 6:9 And he said, Go, and tell this people, Hear ye indeed, but understand not; and see ye indeed, but perceive not. 10 Make the heart of this people fat, and make their ears heavy, and shut their eyes; lest they see with their eyes, and hear with their ears, and understand with their heart, and convert, and be healed.

The Israelites did not understand the words of Jesus, the Christ, according to the prophecy of Isaiah in the Old Testament. (John 12:36-41, Matthew 13:13-17)\

John 12:36 While ye have light, believe in the light, that ye may be the children of light. These things spake Jesus, and departed, and did hide himself from them. 37 But though he had done so many miracles before them, yet they believed not on him: 38 That the saying of Esaias the prophet might be fulfilled, which he spake, Lord, who hath believed our report? and to whom hath the arm of the Lord been revealed? 39 Therefore they could not believe, because that Esaias said again, 40 He hath blinded their eyes, and hardened their heart; that they should not see with their eyes, nor understand with their heart, and be converted, and I should heal them. 41 These things said Esaias, when he saw his glory, and spake of him.

Matthew 13:13 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.

Only those who are chosen by God can understand the mystery of the kingdom of God, that is, that Jesus is the Christ. (Mark 4:10-12, Luke 8:10)

Mark 4:10 And when he was alone, they that were about him with the twelve asked of him the parable. 11 And he said unto them, Unto you it is given to know the mystery of the kingdom of God: but unto them that are without, all these things are done in parables: 12 That seeing they may see, and not perceive; and hearing they may hear, and not understand; lest at any time they should be converted, and their sins should be forgiven them.

Luke 8:10 And he said, Unto you it is given to know the mysteries of the kingdom of God: but to others in parables; that seeing they might not see, and hearing they might not understand.

Paul said that the gospel of God's salvation, that is, the gospel of God's salvation, was preached to the Gentiles because the people of Israel did not realize that Jesus was the Christ. (Rom 11:7, Acts 28:23-28)

Rom 11:7 What then? Israel hath not obtained that which he seeketh for; but the election hath obtained it, and the rest were blinded.

Acts 28:23 And when they had appointed him a day, there came many to him into his lodging; to whom he expounded and testified the kingdom of God, persuading them concerning Jesus, both out of the law of Moses, and out of the prophets, from morning till evening. 24 And some believed the things which were spoken, and some believed not. 25 And when they agreed not among themselves, they departed, after that Paul had spoken one word, Well spake the Holy Ghost by Esaias the prophet unto our fathers, 26 Saying, Go unto this people, and say, Hearing ye shall hear, and shall not understand; and seeing ye shall see, and not perceive: 27 For the heart of this people is waxed gross, and their ears are dull of hearing, and their eyes have they closed; lest they should see with their eyes, and hear with their ears, and understand with their heart, and should be converted, and I should heal them. 28 Be it known therefore unto you, that the salvation of God is sent unto the Gentiles, and that they will hear it.