After his use of Scripture to confirm what has actually happened in his mission, Paul draws out the personal consequences for the Galatian believers: Now you, brothers, like Isaac, are children of promise (v. 28). Just as Isaac was born as the result of a promise, so the Gentile believers were born as a result of the fulfillment of God's promise to Abraham (3:8) and his promise through the prophet Isaiah (4:27). So the link between the Galatians and Isaac is established.
That link is confirmed by the Galatians' experience of persecution. The Jewish Christian teachers have been harassing them with their requirements and demands. That is exactly what happened in the story of Ishmael and Isaac: At that time the son born in the ordinary way persecuted the son born by the power of the Spirit. It is the same now (v. 29). Genesis says that Ishmael mocked Isaac (Gen 21:9). Interpreting this text in the light of his own experience, Paul saw Ishmael's treatment of Isaac as derisive and abusive.
One personal consequence of being like Isaac is being mocked and persecuted by "false brothers" like Ishmael. Paul experienced fierce opposition from "false brothers" who tried to destroy him and his work. As it was at that time . . . it is the same now (v. 29). His story has been repeated many times throughout the history of the church. Often the most painful opposition comes not from those who are totally unrelated to the church, but from those who have positions of power within the church but lack the true power of the Spirit. We can see this illustrated in the time of the Protestant Reformation, when the powers of the Church of Rome ruthlessly persecuted the Reformers.
Now Paul is ready to apply the law directly to the Galatian crisis: But what does the Scripture say? "Get rid of the slave woman and her son" (v. 30). Paul has really turned everything upside down. To those who want to be under the law he gives a command that must be interpreted within his framework of definitions to mean that they should expel the law teachers: Obey the law by getting rid of the law teachers! Excommunicate them!
The command for expulsion also carries with it an exclusion from inheritance: For the slave woman's son will never share in the inheritance with the free woman's son (v. 30). This has sometimes been taken as an absolute exclusion of all Jews, or at least of all unbelieving Jews. But Paul has a more specific target in mind. He is concerned about the subversive influence of those who have been teaching another gospel (1:6-9), those who have been bewitching his converts with their demand for law observance (3:1), those who are zealous to win the Galatians to themselves and to alienate them from Paul (4:17). It is these people who are forfeiting their inheritance by depending on the law rather than on the promise fulfilled in Christ (3:18).
The clear implication of this exclusion of the law teachers from the inheritance is that those who depend on the promises of God fulfilled in Christ will receive the inheritance. They are the true children of Abraham and Sarah; they are the Isaacs.
The consequence of being an Isaac is not only persecution, it is also inheritance. The pain of rejection by "false brothers" is more than offset by the joy of acceptance as children and heirs of promises made and kept by God. Already all who have faith in Christ enjoy the inheritance: they have received their citizenship in the heavenly Jerusalem. The proof of that citizenship is the presence of the Spirit in their lives: they have been born by the power of the Spirit (v. 29).
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