Ephesians 3:5, “[The mystery] was not made known to the sons of men in other generations as it has now been revealed to his holy apostles and prophets by the Spirit.”
Reformed theology emphasizes covenant theology, which means the continuity of God’s work throughout history by means of His covenants with the people of God. When the Lord takes a new step to advance the plan of salvation, the Lord does not ignore or do away with what came before, but rather harmoniously works in accordance with earlier biblical revelation. The Lord might, for example, expand on what He previously said or develop the implications of salvation, more concretely. With that said, His work among His people is essentially the same no matter the covenant that is currently in force (Westminister Confession of Faith 7:5-6). The Lord Jesus fulfills what was revealed before His Advent, but does not abolish, “the Law or the Prophets” (Matthew 5:17-20).
As we consider what we will about the mystery of Christ it’s critical that we keep in mind the above because the mystery of Christ was not made known to earlier generations as it has been in the New Covenant (Ephesians 3:5). It is not as if the Prophets, for example, had no inkling the Messiah would die and open the door for the Gentiles’ inclusion among the people of God. Isaiah, for one example, knew that Christ would atone for the sins of His people (Isaiah 53). Isaiah, also, understood the nations would come to the light of the glory of God in the last days (Isaiah 60:1-3). Isaiah though did not foresee that the Messiah would die in the manner of crucifixion or that the Gentiles would for a time at least, outnumber ethnic Jews in the Church. Additionally, the Prophets did not know the very hour the Messiah would come; in fact, Ephesians 3:5 implies they did not.
With that said, the Old Covenant Prophets had they lived to see the coming of the Lord Jesus, would have recognized; He was the one they spoke of. Simply put, they did not have all the details beforehand. John Calvin says, “There had always been some of the Jewish nation who acknowledged that, at the advent of the Messiah, the grace of God would be proclaimed throughout the whole world, and who looked forward to the renovation of the human race. The prophets themselves, though they spoke with certainty of revelation, left the time and manner undetermined. They knew that some communication of the grace of God would be made to the Gentiles, but at what time, in what manner, and by what means it should be accomplished, they had no information whatever.”
Under the New Covenant, the nature of fuller revelation means that Christians must always interpret the Old Testament in light of the New Testament. Christians must recognize, they are to read the Old Covenant revelation through the lens of the New Covenant, which is why the Church prizes the Epistles.