The law and the prophets speak of Christ. God spoke from the beginning of the world by the mouth of His holy prophets, progressively revealing His purpose for the nation of Israel and the earth. Through Israel's rise, the whole world would be blessed.
But Israel is currently fallen and blind, and Gentiles would be without hope if it were not for a secret that God had kept. The mystery of Christ concerns the unsearchable riches of Christ. Not spoken by the prophets, hid in God, revealed to the apostle Paul.
Recognizing the differences between the contents of prophecy and mystery is vital in understanding who we are so that we may serve the Lord worthily.