All the NT writers quote OT scripture, quote that Jesus taught OT scripture, and his disciples used the OT scriptures to witness Jesus Christ as Messiah to Jews. You just simply cannot sweep the OT under the rug.
And not all prophecies are fulfilled. Luke wrote both Luke and Acts to prove this point at the simplest level.
**Luke 24:44-45
He said to them, “This is what I told you while I was still with you: Everything must be fulfilled that is written about me in the Law of Moses, the Prophets and the Psalms.”
Then he opened their minds so they could understand the Scriptures.
Acts 1:6-7
So when they met together, they asked him, “Lord, are you at this time going to restore the kingdom to Israel?”
He said to them: "It is not for you to know the times or dates the Father has set by his own authority. **
Israel will be fully restored when Jesus comes back to Israel a 2nd time. The OT is full of prophecies concerning such.
No way. No how. In the New Testamnet, the Gospel is to the Jew first. So how do you figure then you can cut out the scriptures they use, which is the OT?
That authority would be Jesus Christ,
see Luke 24:44:45.
Also note that St.Peter wrote in his epistle about the transfiguaration of Jesus, what he called " the power and coming of our Lord Jesus Christ". He then adds this to his account-
**1 Peter 1:16-21
We did not follow cleverly invented stories when we told you about the power and coming of our Lord Jesus Christ, but we were eyewitnesses of his majesty. For he received honor and glory from God the Father when the voice came to him from the Majestic Glory, saying, "This is my Son, whom I love; with him I am well pleased."We ourselves heard this voice that came from heaven when we were with him on the sacred mountain.
And we have the word of the prophets made more certain, and you will do well to pay attention to it, as to a light shining in a dark place, until the day dawns and the morning star rises in your hearts. Above all, you must understand that no prophecy of Scripture came about by the prophet’s own interpretation. For prophecy never had its origin in the will of man, but men spoke from God as they were carried along by the Holy Spirit.**
Did you catch that?-" And we have the word of the prophets made more certain, and you will do well to pay attention to it,…
** Yes, The Old Testament is extremely important today.**