27. For it is written. The apostle proves, by a quotation from Isaiah, that the lawful sons of the Church are born according to the promise. The passage is in Isaiah 54 where the prophet speaks of the kingdom of Christ and the calling of the Gentiles, and promises to the barren wife and the widow a numerous offspring; for it is on this ground that he exhorts the Church to “sing” and “rejoice.” The design of the apostle, let it be carefully remarked, is to deprive the Jews of all claim to that spiritual Jerusalem to which the prophecy relates. Isaiah proclaims, that her children shall be gathered out of all the nations of the earth, and not by any preparation of hers, but by the free grace and blessing of God.
He next concludes that we become the sons of God by promise, after the example ( κατὰ ᾿Ισαὰκ) of Isaac, and that in no other way do we obtain this honor. To readers little skilled or practiced in the examination of Scripture, this reasoning may appear inconclusive; because they do not hold the most undoubted of all principles, that all the promises, being founded on the Messiah, are of free grace. It was because the apostle took this for granted, that he so fearlessly contrasted the promise with the law.