What did the Law of Moses and the prophets teach about?
The Law of Moses and the prophets testified to the righteousness of God that is now manifested apart from the Law. They also taught concerning the coming King and His Kingdom, forming the foundation on which grace rests as part of God's progressive revelation.
Les explains in Romans chapter 3 that the righteousness of God without the Law was witnessed by the Law and the prophets. He emphasizes that the Bible is a progressive revelation: the Law of Moses came first, then the prophets with their teachings concerning the coming King and the Kingdom. Even though in this age of grace nothing in our salvation comes from the Law, grace rests on that earlier foundation. Les notes that when those things fell through the cracks, God temporarily turned to the Gentiles and called out the Church, which builds on everything that went before.