Col 1:26-27, Col 2:2, 1 Cor 2:7-10, 1 Jn 1:1-2
What Paul wrote earlier is that the Christ prophesied in the Old Testament is Jesus. The apostles and prophets of the early church were able to understand this mystery through the Holy Spirit. (Eph 3:3-5, 1 Cor 2:7-10)
Eph 3:3 How that by revelation he made known unto me the mystery; (as I wrote afore in few words, 4 Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) 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;
1 Cor 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.
The mystery that Jesus is the Christ, long hidden, is now revealed to the saints. (Col 1:26-27, 1 Jn 1:1-2)
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:
1 Jn 1:1 That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have handled, of the Word of life; 2 (For the life was manifested, and we have seen it, and bear witness, and shew unto you that eternal life, which was with the Father, and was manifested unto us;)
We must understand the mystery of Christ throughout the Bible. (Col 2:2)
Col 2:2 That their hearts might be comforted, being knit together in love, and unto all riches of the full assurance of understanding, to the acknowledgement of the mystery of God, and of the Father, and of Christ;