2 Tim 3:15, Lk 24:25-27, 44, Acts 8:34-35, Acts 10:43, Acts 13:15, Acts 13:22-23, 27, Acts 26:22, Rom 16:25-26,

The apostle John explains the purpose of writing the Bible. The purpose of the Bible is to believe in Jesus as the Christ and to gain life. (John 20:31)

Jn 20:31 But these are written, that ye might believe that Jesus is the Christ, the Son of God; and that believing ye might have life through his name.

Paul explains to his disciple Timothy that the Old Testament has wisdom that leads to salvation by believing that Jesus is the Christ. (2 Timothy 3:15)

2 Tim 3:15 And that from a child thou hast known the holy scriptures, which are able to make thee wise unto salvation through faith which is in Christ Jesus.

Jesus also explained to His disciples that the Old Testament was describing Christ and that that Christ was Jesus. (Luke 24:25-27,44)

Lk 24:25 Then he said unto them, O fools, and slow of heart to believe all that the prophets have spoken: 26 Ought not Christ to have suffered these things, and to enter into his glory? 27 And beginning at Moses and all the prophets, he expounded unto them in all the scriptures the things concerning himself. 44 And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.

Philip explained the book of Isaiah to the Ethiopian eunuch, who was reading the Old Testament book of Isaiah, but did not know who the person speaking in Isaiah was, and explained that the one Isaiah was talking about was Jesus, the Christ. (Acts 8:34-35)

Acts 8:30 And Philip ran thither to him, and heard him read the prophet Esaias, and said, Understandest thou what thou readest? 31 And he said, How can I, except some man should guide me? And he desired Philip that he would come up and sit with him. 32 The place of the scripture which he read was this, He was led as a sheep to the slaughter; and like a lamb dumb before his shearer, so opened he not his mouth: 33 In his humiliation his judgment was taken away: and who shall declare his generation? for his life is taken from the earth. 34 And the eunuch answered Philip, and said, I pray thee, of whom speaketh the prophet this? of himself, or of some other man? 35 Then Philip opened his mouth, and began at the same scripture, and preached unto him Jesus.

Peter testified that all the prophets of the Old Testament testified of Christ and that that Christ was Jesus. (Acts 10:43)

Acts 10:43 To him give all the prophets witness, that through his name whosoever believeth in him shall receive remission of sins.

When the rulers of the synagogue asked for explanations on this part after reading the Law and the Prophets, Paul explained the parts and testified that Jesus was the Christ. (Acts 13:15,22-23,27)

Acts 13:15 And after the reading of the law and the prophets the rulers of the synagogue sent unto them, saying, Ye men and brethren, if ye have any word of exhortation for the people, say on. 22 And when he had removed him, he raised up unto them David to be their king; to whom also he gave their testimony, and said, I have found David the son of Jesse, a man after mine own heart, which shall fulfil all my will. 23 Of this man's seed hath God according to his promise raised unto Israel a Saviour, Jesus: 27 For they that dwell at Jerusalem, and their rulers, because they knew him not, nor yet the voices of the prophets which are read every sabbath day, they have fulfilled them in condemning him.

When Paul became a prisoner and was tried, he testified that the prophecy about Christ in the Old Testament that he would surely be fulfilled was fulfilled in Jesus. (Acts 26:22)

Acts 26:22 Having therefore obtained help of God, I continue unto this day, witnessing both to small and great, saying none other things than those which the prophets and Moses did say should come:

As Paul concluded the book of Romans, he once again testified that the Christ prophesied by the prophets of the Old Testament was Jesus, and blessed the Roman church in the name of that Christ, Jesus. (Romans 16:25-26)

Rom 16:25 Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began, 26 But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith: