Paul's argument moves from experience to Scripture. He reminds the foolish Galatians that Christ crucified was vividly portrayed before them. They received the Spirit by hearing with faith, not by works of the law. Having begun by the Spirit, would they now seek completion by the flesh? God supplies the Spirit and works miracles among them through faith.
Abraham believed God, and it was credited to him as righteousness, citing Genesis 15:6. Therefore those of faith are Abraham's true children. Scripture foresaw that God would justify the Gentiles by faith, announcing beforehand that all nations would be blessed in Abraham. All who rely on works of the law are under a curse, for the law demands complete obedience, while Habakkuk 2:4 says the righteous will live by faith. Christ redeemed believers from the curse of the law by becoming a curse for them, as Deuteronomy 21:23 describes one who hangs on a tree. So the blessing of Abraham comes to the Gentiles, and believers receive the promised Spirit through faith.
A covenant once ratified cannot be annulled, and the promise to Abraham and his offspring, singular, points to Christ. The law, coming 430 years later, cannot void it. The law was added because of transgressions until the offspring came, and it served as a guardian, like a household slave who supervised children until they came of age. Now that faith has come, believers are no longer under a guardian. They are all sons of God through faith, having been baptized into Christ and clothed with Him. Ethnic, social, and gender distinctions no longer determine standing before God, for all are one in Christ Jesus and heirs according to the promise.