1 Cor 2:7-10, Eph 3:5-6, Col 1:26-27
In the Old Testament, God did not allow people to understand God even if they heard the word of God. (Isa 64:4)
Isa 64:4 For since the beginning of the world men have not heard, nor perceived by the ear, neither hath the eye seen, O God, beside thee, what he hath prepared for him that waiteth for him.
The mystery of God, the wisdom of God, is Christ Jesus. (Col 1:26-27)
Col 1:26 Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints: 27 To whom God would make known what is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory:
God did not allow people to understand Christ, the wisdom of God, but only through the Holy Spirit. (1 Corinthians 2:7-10; Ephesians 3:5-6)
1 Corinthians 2:7 But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory: 8 Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory. 9 But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him. 10 But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.
Ephesians 3:5 Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit; 6 That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel: