The preaching of the Old Testament is not a dirge that must end with a happy chord. The gospel of Jesus Christ is the melody of the scriptures from beginning to end. Apostolic preaching will always demonstrate this.
Acts 9v22 - Yet Saul grew more and more powerful and baffled the Jews living in Damascus by proving that Jesus is the Christ.
Acts 17v2-4 - As his custom was, Paul went into the synagogue, and on three Sabbath days he reasoned with them from the Scriptures, explaining and proving that the Christ had to suffer and rise from the dead. "This Jesus I am proclaiming to you is the Christ"
Acts 18v28 - For he vigorously refuted the Jews in public debate, proving from the Scriptures that Jesus was the Christ.
This was Paul's evangelistic approach. Paul with his Old Testament proving, reasoning, refuting, explaining and proving that their true meaning is the gospel of Jesus.
It could be argued that this is his approach among Jews - which it was. And granted at Athens he seems to argue more from creation and culture. But when we consider his approach to the Gentiles of Galatia he says he 'portrayed Christ crucified'. He promptly goes on to spend several verses reminding them about Abraham and the law. It seems he'd quickly shown them that the gospel preached to Abraham 'all peoples will be blessed through you' was the meaning of Christ crucified.
If we only operate with a 27 book Bible we rob ourselves of the roots and foundations on which the gospel stands. Our gospel will be flimsy and appear 'new' when it is rooted in everything God has ever said, the God who reveals all this plans. Amos 3v7: For the Lord GOD does nothing without revealing his secret to his servants the prophets. That's spoken into a context where God's people were silencing their prophets, and so robbing themselves of the life-giving knowledge of God. We do the same to ourselves if we silence the law and the prophets by not permitting them to sing their true melody, about the Christ.
Posted by Dave Bish. More thoughts on how to read the Old Testament as about Jesus can be found at The Biblical Theology Briefings.