Today's New Testament reading is John 5:30-47. In that section, our Lord says, "You search the Scriptures because you think that in them you have eternal life; and it is they that bear witness about me" (John 5:39).
At the Transfiguration of our Lord, Moses and Elijah appeared with Christ on the Mount talking with him about his coming death and resurrection. Their appearance testified to Christ being the center of the Law (Moses) and the Prophets (Elijah). Jesus in our verse above says that the Scriptures (meaning, the Old Testament) bear witness to him. Paul writes, (2 Timothy 3:14-15) "But as for you, continue in what you have learned and have firmly believed, knowing from whom you learned it and how from childhood you have been acquainted with the sacred writings [the Old Testament], which are able to make you wise for salvation through faith in Christ Jesus."
No wonder the Father said on the Sacred Mountain: "This is my Son, whom I love; with him I am well pleased. Listen to Him!"
Augustine wrote: "The New Testament is hidden in the Old Testament, and the Old Testament is made clear in the New Testament" (TLSB, page 1).
Luther, in his "Prefaces to the Old Testament" wrote: "There are some who have little regard for the Old Testament...But Christ says in John 5{:39], "Search the Scriptures, for it is they that bear witness to me."...The Scriptures of the Old Testament are not to be despised, but diligently read. For [the apostles] base the New Testament upon them mightily" (TLSB, page 1).
Finally, in one of his Christmas Day sermons, Luther wrote: "We should know that all that the apostles taught and wrote, they took out of the Old Testament; for in it all things are proclaimed that were to be fulfilled later in Christ, and were to be preached, as Paul says in Rom. 1,2: 'God promised afore the Gospel of his Son Jesus Christ, through his prophets in the Holy Scriptures.' Therefore all their preaching is based upon the Old Testament, and there is not a word in the New Testament that does not look back into the Old, where it had been foretold."
What, then, are the OT Scriptures about? Jesus!
"Fulfiller of the past, Promise of things to be, We hail your body glorified And our redemption see" Amen (CW 95:3).