The Galatian Judaizers thought their argument for Abraham as one justified by faith and works was convincing, but Paul has proved to be their superior in interpreting Scripture. There is no way that sinners can be declared righteous in God’s sight by doing the works of the Law because Abraham was justified before he was circumcised, and he did not live under the Mosaic law (Gen. 15:1–6). Augustine says, “If the law justifies, Abraham was not justified, since he lived long before the law” (Ancient Christian Commentary, NT vol. 8, p. 43).
Being declared righteous is the way people, Jew and Gentile alike, are counted as sons and daughters of Abraham and thus, heirs of the inheritance promised to the patriarch (Gal. 3:6–7). But what exactly is this inheritance? To answer this question we must turn to Genesis, wherein we read that God promised Abraham a good land, many offspring, a blessed life that would be a blessing to the world, and victory over His enemies (12:1–3; 22:15–18). Our Creator Himself pledged to do all that is required to bring this inheritance about. When He appeared to Abraham as a smoking pot and flaming torch that passed between the corpses of animals, God swore that His plan for the patriarch’s descendants would be accomplished or He would die just like those animals (Gen. 15:7–21). Since God cannot die, it is impossible for His promise to fail.
In Jesus, our Creator did all the work necessary to bring these things to pass. He defeated Satan, initiated the renewal of all things, and commissioned His church to declare His gospel that the world might be blessed through the addition of all nations to the family of Abraham (Matt. 4:1–11, 23–25; 28:18–20). The Father promised His Son that all this would be accomplished if He would trust and submit to God’s plan. Our Savior never doubted Him, living and dying to fulfill all that the Scriptures foretold (26:36–56).
Galatians 3 does not describe the promised inheritance in full, only the Spirit of promise (v. 14). But justification gives us the inheritance, and the indwelling of the Spirit provides proof of justification (v. 2); thus, the Spirit is shorthand for the fact that by faith we receive everything pledged to Abraham.