v. 28. Now we, brethren, as Isaac was, are the children of promise.
v. 29. But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now.
v. 30. Nevertheless, what saith the Scripture? Cast out the bond-woman and her son; for the son of the bond-woman shall not be heir with the son of the freewoman.
v. 31. So then, brethren, we are not children of the bond-woman, but of the free.
What the prophet had foretold found its realization in the Church of the New Testament. We Christians, we believers, who by virtue of the Gospel-promise are spiritual descendants of Abraham, are, after the manner of Isaac, children of the promise. See 1Pe_1:15. As Isaac was born to Abraham by virtue of the promise of God, so we, through faith in the promise of the Gospel, have been born anew, by the Spirit of God, as His true children, Rom_9:8.
History, however, repeats itself: Just as then he that was born after the flesh made it a practice to persecute the one after the Spirit, thus it is also now. In addition to the fact that Ishmael, Gen_21:9, is called a scoffer, we here have the information that his attitude toward Isaac was that of continual nagging, boasting, and despising, a persecution all the more diabolical since it left little tangible cause for which one could call the lad to account. Isaac had been born according to the Spirit; the Spirit of God through His creative power had restored the atrophied organs of Sarah, Isaac's birth thus being a miracle. Undoubtedly this fact had much to do with the attitude of Ishmael. But the very same disposition, the apostle says, we find in the world today; those that are in bondage under the Law and are firmly convinced that they can earn the inheritance of heaven by their works are full of hatred and enmity against those that rely upon the gracious promise of God; the self-righteous and hypocrites despise and persecute the true Christians.
But the confidence and supercilious attitude of the self-righteous will not be of long duration. For what does the Scripture say? Cast out the slave and her son; for the son of the slave shall not be heir together with the son of the free-woman. Gen_21:10. That was Sarah's decision with reference to Hagar and Ishmael, the bond-woman and her offspring. And such is God's sentence upon all those that are the spiritual descendants of Ishmael, that attempt to get to heaven by the works of the Law. In spite of the persecution to which the believers in the Gospel-promises are subjected, they have on their side the divine assurance that the plans of their enemies will be unsuccessful in the end, that they have no part in the blessings of the Kingdom of Grace and the Kingdom of Glory, for they cannot be heirs together with the children of the Spirit.
So Paul concludes triumphantly: Wherefore, brethren, we are not children of the bond-woman, but of the free. We Christians that believe in Christ and hope by such faith to be justified before God and obtain eternal life, are not among those who, like Hagar and Ishmael, were expelled from the home of Sarah and Abraham, but by the grace of God belong to those that are typified by Isaac, the son of promise; we are true children of God and heirs of eternal life. Thus the fact which Paul intended to set forth in this allegory stands out beyond contradiction over against the Judaizing teachers: The religion of the Law brings into bondage and thus to final condemnation; only the Gospel liberates and brings salvation, and therefore the religion of the Gospel is the only true religion.
Paul reminds the Galatians that they are children of God, and that they should therefore not return to the bondage of the Law; he rebukes them for falling may from their former loving relation toward him, which, he says, is due to the sordid methods of the false teachers; he shows in an allegory that only the children of the promise will obtain salvation.