To give a human example, brothers: even with a man-made covenant, no one annuls it or adds to it once it has been ratified. Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” referring to many, but referring to one, “And to your offspring,” who is Christ. This is what I mean: the law, which came 430 years afterward, does not annul a covenant previously ratified by God, so as to make the promise void. For if the inheritance comes by the law, it no longer comes by promise; but God gave it to Abraham by a promise. – Galatians 3:15-18 ESV
As a former Pharisee, Paul had a scholarly understanding of the Old Testament. But it was after his conversion, when he had received the indwelling Holy Spirit, that Paul truly began to understand that the Old Testament was a foreshadowing of the coming Messiah, Jesus Christ. Post-conversion, his comprehension of the Scriptures was both magnified and clarified. Familiar passages took on a whole new meaning when he was able to view to view them through the lens of the gospel. In the case of Genesis 13:15 and 17:8, where Moses records God’s covenant to Abraham, Paul exegetes these all-too-familiar passages by revealing that through them, God had been promising the coming of Christ. He was the “seed” or “offspring” through whom all the nations would be blessed. God’s promise to Abraham would ultimately be fulfilled through Jesus. But what is Paul’s point in bringing this new interpretation or understanding of God’s promise to light? He was attempting to answer the argument that the Mosaic law, which came after the giving of the promise by God to Abraham, somehow superceded or supplanted it.
On the contrary, Paul argues, God had made a binding covenant with Abraham and his “offspring.” That God-ordained covenant could not be nullified or broken. It was a unilateral covenant, made by God and could only be annulled by God. But Paul argues that at no point did God revoke or replace the covenant, even when He had given Moses the law some 430 years afterwards. In fact, Paul points out, the inheritance tied to the promise of God could be received only through the promise of God. It was not accessible any other way, especially not through the keeping of the law. And the inheritance of which Paul speaks is tied directly to the idea of justification by faith. This was the crux of the problem taking place among the Galatian believers. They were being told that their justification was tied to the keeping of the law, most specifically to God’s command regarding circumcision. In other words, they were being sold a bill of goods that promised them a right standing before God through law-keeping and self-effort, not faith in Christ alone.
Paul wrote to the Colossian believers:
May you be strengthened with all power, according to his glorious might, for all endurance and patience with joy, giving thanks to the Father, who has qualified you to share in the inheritance of the saints in light. He has delivered us from the domain of darkness and transferred us to the kingdom of his beloved Son, in whom we have redemption, the forgiveness of sins. – Colossians 1:11-14 ESV
He prayed for the Ephesian…
that the God of our Lord Jesus Christ, the Father of glory, may give you the Spirit of wisdom and of revelation in the knowledge of him, having the eyes of your hearts enlightened, that you may know what is the hope to which he has called you, what are the riches of his glorious inheritance in the saints… – Ephesians 1:17-18 ESV
For Paul, the promise of God made to Abraham and fulfilled in Christ, was all about the wonderful reality of a restored relationship with God, for both Jews and Gentiles. And this incredible gift was only available through faith in the finished work of Jesus Christ. It could not be attained through self-effort. Paul went on to tell the Ephesian believers that he wanted them to understand
what is the immeasurable greatness of his power toward us who believe, according to the working of his great might that he worked in Christ when he raised him from the dead and seated him at his right hand in the heavenly places… – Ephesians 1:19-20 ESV
Our salvation, justification, sanctification and glorification are all tied to the work of Christ on our behalf. Our righteousness comes from Christ. Our right standing before God is as a result of His shed blood. Our future inheritance is tied His sacrificial death on the cross. All that we are and all that we hope to have is based on the finished work of Christ. And for that we have much to be grateful.