Reformed theology is covenant theology, meaning that it places stress on the continuity of God’s work throughout history by means of His covenants with His people. In other words, when the Lord takes a new step to advance His plan of salvation, He does not ignore or do away with what came before, but rather works in a way that is harmonious with earlier revelation. God might expand on what He said previously or develop the implications of His salvation more concretely, but His work among His people is essentially the same no matter the covenant that is currently in force (WCF 7:5–6). Christ fulfills what was revealed before His advent; He does not abolish “the Law or the Prophets” (Matt. 5:17–20).
We must keep this in mind when we consider the idea that the mystery of Christ was not made known to earlier generations as it has been revealed in the new covenant (Eph. 3:5). It is not as if the prophets had no inkling that the Messiah would die and open the door for the Gentiles’ inclusion among God’s people. Isaiah, for example, understood that the Christ would atone for the sins of His own (Isa. 53). He also knew the nations would come to the light of God’s glory in the last days (60:1–3). But he did not necessarily foresee in detail that the Messiah would die via crucifixion or that the Gentiles would, for a time at least, outnumber ethnic Jews in the church. We need not believe that the prophets knew the very hour at which the Messiah would come. In fact, today’s passage implies they did not (Eph. 3:5).
Had the old covenant prophets lived to see the coming of the Christ, they would have recognized that He was the one of whom they had been speaking all along. They just did not have all the details filled out beforehand. John Calvin comments, “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.”