In the time of the apostles, the Old Testament was their Bible. It wasn’t until after they died that their own writings were collected, assembled into what is called the New Testament, and added to the Bible. Therefore, whenever you see the apostles referring to, or quoting the Scriptures, they are talking about what we call the Old Testament.
The apostles had a particular way of reading the Old Testament. It was the way Jesus had taught them. In the last chapter of Luke, Jesus makes clear to His apostles (and Luke makes clear to his readers) that the Old Testament was full of information about Jesus. The apostles, like all good Jews of that day, had previously seen the Old Testament as a collection of law, history, and some prophecies about Israel including some references to a Messiah. In the light of Jesus’ resurrection, however, they came to see almost the entirety of the Old Testament as describing the work of Messiah.
Through the gospels and letters of the New Testament, we see examples of how the apostles came to see Jesus in the Old Testament by the way they quoted it. They didn’t merely see Him in the obvious prophecies that had been made about Messiah being born in the line of David. They saw Him as the Passover that Israel celebrated, the manna in the desert, the rock that followed them, the serpent that Moses lifted, the ladder that Jacob had dreamed, the Shepherd that David extolled, the temple that Solomon built, the wisdom that the Queen of Sheba came to hear, and much more. In fact, even the entire New Testament itself is incapable of explicitly reporting every reference to Jesus in the Old Testament. The left us the path, however, to finding the rest.
Learning about Jesus nourishes our faith. Why should we study anything else in the Old Testament except to look for Jesus?
The apostles had a particular way of reading the Old Testament. They abandoned the historic method of reading it according to the flesh. Instead, they read it in the light of what they called the grace of our Lord Jesus Christ. We would do well to read it the same way.