“making known to us the mystery of His will, according to His good pleasure which He purposed in Him as a plan for the fullness of time to sum up all things in Christ — that which is in the heavens and that which is on the earth, in Him.”
Thus, it is by God’s grace that He makes known to us the mystery of His will. To what, then, does this mystery refer? The simple answer is that the mystery of God’s will has to do with God’s plan of redemption through his Son. One might respond by asking, “How is this a mystery? Surely we find the Gospel bound in the Old Testament as well as in the new?” It is very true that there are pointers to the Gospel of Jesus Christ throughout the Scriptures. These pointers even begin with the Fall in the Garden where God promises that a redeemer would come (Genesis 3:15).
The Gospel is found in bits and pieces, then, throughout the Old Testament. It can be found in the various aspects of the covenant made with Noah, Abraham, Isaac, and Jacob. It is found in people like Boaz, who point us to Christ. It is found in the institutions of the Judge, the Prophet, the Priest, and the King, and it is found in more explicit ways, like in Isaiah 53 or in Psalm 103.
At the same time, we have the advantage of hindsight when we look at these events and peoples. We understand what the Author of Hebrews clearly states, that indeed, all of these things point us to Christ. We can look back and see the Christ for whom the saints of the Old Testament diligently searched (1 Peter 1:10-12). As Sherlock Holmes would argue, when you know the elements that lead up to an event, discerning the solution is a matter of elementary deduction. We have the advantage of being able to “reason backwards” as Holmes would call it, and doing so, the mystery is not a mystery to us — God has revealed it in his Son. At the same time, those who lived on the other side of the cross had a much more difficult time understanding the nature of these prophetic writings that formed clues to the solution revealed in Christ.