When the Bible speaks of "the Law and the Prophets," it refers to the Old Testament. Peter preached from the Old Testament on the day of Pentecost, and three thousand people received the Holy Spirit (Acts 2:17, 25, 34). "The ministry of the Spirit" (2 Cor 3:8) began by the preaching of the ministry of the law.
Jesus' Transfiguration featured the Law, represented by Moses, and the Prophets, represented by Elijah (Lk 9:30). When Jesus was transfigured into a greater glory (Lk 9:29ff), He didn't just bypass the old covenant, He fulfilled it (Mt 5:17). Thus the presence of Moses and Elijah at His Transfiguration shows the Old Testament has a place in the New Testament's ministry of the Spirit.
On Easter evening, Jesus changed the hearts of His two disciples on the road to Emmaus by opening the Scriptures to them, which at that point consisted of only the Old Testament. Jesus began with "Moses and all the prophets" and "interpreted for them every passage of Scripture which referred to Him" (Lk 24:27).
Jesus declared that the Old Testament is more capable of effecting change on a hard heart than would be the sight of a man risen from the dead! (Lk 16:31) Read the Old Testament. Then teach the Old Testament to a hard-hearted world (Mt 5:19).