21 Tell me, ye that desire to be under the law, do ye not hear the law?   22 For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman.   23 But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise.   24 Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar.   25 For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children.   26 But Jerusalem which is above is free, which is the mother of us all.   27 For it is written, Rejoice, thou barren that bearest not; break forth and cry, thou that travailest not: for the desolate hath many more children than she which hath a husband.   28 Now we, brethren, as Isaac was, are the children of promise.   29 But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now.   30 Nevertheless what saith the scripture? Cast out the bondwoman and her son: for the son of the bondwoman shall not be heir with the son of the freewoman.   31 So then, brethren, we are not children of the bondwoman, but of the free.

      In these verses the apostle illustrates the difference between believers who rested in Christ only and those judaizers who trusted in the law, by a comparison taken from the story of Isaac and Ishmael. This he introduces in such a manner as was proper to strike and impress their minds, and to convince them of their great weakness in departing from the truth, and suffering themselves to be deprived of the liberty of the gospel: Tell me, says he, you that desire to be under the law, do you not hear the law? He takes it for granted that they did hear the law, for among the Jews it was wont to be read in their public assemblies every sabbath day; and, since they were so very fond of being under it, he would have them duly to consider what is written therein (referring to what is recorded Gen. xvi. and xxi.), for, if they would do this, they might soon see how little reason they had to trust in it. And here, 1. He sets before them the history itself (v. 22, 23): For it is written, Abraham had two sons, &c. Here he represents the different state and condition of these two sons of Abraham--that the one, Ishmael, was by a bond-maid, and the other, Isaac, by a free-woman; and that whereas the former was born after the flesh, or by the ordinary course of nature, the other was by promise, when in the course of nature there was no reason to expect that Sarah should have a son. 2. He acquaints them with the meaning and design of this history, or the use which he intended to make of it (v. 24-27): These things, says he, are an allegory, wherein, besides the literal and historical sense of the words, the Spirit of God might design to signify something further to us, and that was, That these two, Agar and Sarah, are the two covenants, or were intended to typify and prefigure the two different dispensations of the covenant. The former, Agar, represented that which was given from mount Sinai, and which gendereth to bondage, which, though it was a dispensation of grace, yet, in comparison of the gospel state, was a dispensation of bondage, and became more so to the Jews, through their mistake of the design of it, and expecting to be justified by the works of it. For this Agar is mount Sinai in Arabia (mount Sinai was then called Agar by the Arabians), and it answereth to Jerusalem which now is, and is in bondage with her children; that is, it justly represents the present state of the Jews, who, continuing in their infidelity and adhering to that covenant, are still in bondage with their children. But the other, Sarah, was intended to prefigure Jerusalem which is above, or the state of Christians under the new and better dispensation of the covenant, which is free both from the curse of the moral and the bondage of the ceremonial law, and is the mother of us all--a state into which all, both Jews and Gentiles, are admitted, upon their believing in Christ. And to this greater freedom and enlargement of the church under the gospel dispensation, which was typified by Sarah the mother of the promised seed, the apostle refers that of the prophet, Isa. liv. 1, where it is written, Rejoice, thou barren that bearest not; break forth and cry, thou that travailest not; for the desolate hath many more children than she who hath a husband. 3. He applies the history thus explained to the present case (v. 28); Now we, brethren, says he, as Isaac was, are the children of the promise. We Christians, who have accepted Christ, and rely upon him, and look for justification and salvation by him alone, as hereby we become the spiritual, though we are not the natural, seed of Abraham, so we are entitled to the promised inheritance and interested in the blessings of it. But lest these Christians should be stumbled at the opposition they might meet with from the Jews, who were so tenacious of their law as to be ready to persecute those who would not submit to it, he tells them that this was no more than what was pointed to in the type; for as then he that was born after the flesh persecuted him that was born after the Spirit, they must expect it would be so now. But, for their comfort in this case, he desires them to consider what the scripture saith (Gen. xxi. 10), Cast out the bond-woman and her son, for the son of the bond-woman shall not be heir with the son of the free-woman. Though the judaizers should persecute and hate them, yet the issue would be that Judaism would sink, and wither, and perish; but true Christianity should flourish and last for ever. And then, as a general inference from the whole of the sum of what he had said, he concludes (v. 31), So then, brethren, we are not children of the bond-woman, but of the free.