Has the Spirit come by the works of the Law or by hearing with faith? Begun by the Spirit, are you now perfected by the works of the flesh? The antithesis here is not “the Law is bad or makes me feel bad” versus “the Gospel is good.” Paul himself rejects such a notion elsewhere (Romans 7:7 ff.). The works of the law which Paul rejects is a focus on one’s own performance of the Law, which, not incidentally, excludes God from the picture. To begin in the Spirit and end in the flesh is to begin with God and end in the self, something which even Abraham did not do.
Justification comes by faith, and Paul reminds the Galatians that God Himself preached the Gospel to Abraham, long before Sinai. “In you shall all the nations be blessed” (Galatians 3:8; Genesis 12:3). Even in the particular blessing of Abraham, the Lord has a greater purpose in mind. Israel, through Abraham, will be the means of blessing the whole earth. Israel is the vehicle of a greater blessing, and the peculiar holiness of Israel serves as a witness to that end.
This is what Israel failed to understand. Israel is called to be holy so that all the nations would come to know the Lord (Isaiah 2:1-5, 1 Kings 4:20-24, Psalm 72:8-11, among others). Yet Israel had perverted her witness to the world into something else entirely. Faith was no longer needed, because they regarded their own separation from the world as their righteousness. Without faith, such separation could only produce hypocrisy and wickedness. “To the wicked God says: ‘What right have you to recite my statutes or take my covenant on your lips’” (Psalm 50:16; also Jeremiah 7:4, Amos 8:4-6, etc.)?
But the righteous shall live by faith. These words of Habakkuk show that the promise made to Abraham has not been set aside. Mamre has not given way to Sinai. Faith is not perfected by the flesh. The promise made to Abraham finds its end in Jesus, through Whom the blessing of Abraham comes also to the whole earth. The Gospel preached to Abraham does not end in Israel, as if the uniqueness of Israel was the whole point. The Gospel goes out to all by faith.
Paul uses a couple of examples to drive his point home here. No one changes human covenants after they have been ratified. If no one tampers with human ones, how much less ought we to tamper with divine ones, especially the one ratified in Genesis 15? More than this, Paul points to the text of Genesis 12 directly, noting that the offspring of Abraham is singular, which can only mean Christ. Were this single word plural, the argument of his opponents might have some weight. Then the inheritance of the physical land, the peculiarity of Israel, might very well be the whole point. But it is not plural, but singular. The scope of the promise made to Abraham is worldwide. The promised inheritance depends on faith and faith alone.
But the Law given at Sinai was put in place because of “transgressions” and also as a “guardian.” Israel broke the covenant when she broke faith with the Lord, setting up the golden calf. The Lord departed from the camp, separating Himself from Israel (Exodus 32-33). Following the intercession of Moses, the Lord remakes the covenant with Israel, but now the veil of Moses covers his face. The transgression of Israel required that veil, because the external letter brought with it the curse (2 Corinthians 3:4-11; Deuteronomy 28-29). But in Jesus Christ, the veil is taken away, because He is the promised offspring. Moses, the intermediary, served to put this into place, but now the promise by faith puts an end to all intermediaries. Delighting in the Law of the Lord (Psalm 1), the Christian inherits the promise of Abraham through the inward working of the Spirit.