Ver. 24. Which things are a typical history; for these two women represent the two covenants, with the churches thereto adhering respectively; the one from Mount Sinai, to wit, the covenant of works, which was given there, and where the Jewish synagogue, under that covenant, had its beginning: this bringeth forth children indeed, but such children as are mere mercenary worshippers, and under the bondage of that covenant, its curses and commands; and this is that represented by Agar the bondwoman, and that very conveniently. Ver. 25. (Gr. To gar Agar, Sina oros estin en te Arabia, &c.), For as to that allegorical Agar, the mountain Sinai is in Arabia, where lived Agar and her son; and the same Sinai answers well to the earthly Jerusalem, the now Jewish synagogue, nothing being heard from either of them, but Do and Live: but Sinai still but serves with the legalists her children, Sinai and her covenant, because it was never designed but to be subservient to the covenant of grace, as Hagar to Sarah; her children, the legalists, following the state of the venter i. e. the womb from whence they came, are but in the state of bond-servants; not in the state of sons and heirs, more than Ishmael had Isaac’s privilege.

“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 an husband.”

Ver. 26. But the other covenant, the covenant of grace, with the church adhering thereto, to wit, the church of believers in Christ, which is the heavenly Jerusalem, enjoying the reality of all the typical external privileges which the earthly Jerusalem had, which covenant and church is the mother of us all, Jews or Gentiles, that believe in Christ, even she is free: the covenant is served, but serves not as Sarah; the church is mistress of the family, being Christ’s spouse; and her children sons and heirs, as Sarah and her Isaac. Ver. 27. I say, the Christian church is that free mother of us all; for Isaiah in his 54th chap. ver. 1. with an eye to that typical history of Hagar and Sarah, in Abraham’s family, after he had spoke of the sufferings of Christ, immediately addressseth himself to her, as she was in her very first days before the preaching of the gospel by the apostles, saying, Rejoice thou that, as Sarah, art yet barren, thy children making no figure in comparison of the synagogue, break forth over all thy discouragements, and cry for joy, though thou travailest not yet to bring forth children, as thou shalt do ere long by the preaching of the apostles, after the effusion of the Holy Ghost on them: for though thou seem desolate as a widow, thy husband being yet in his state of humiliation, and so very little known, thou shalt have many more children, even such as shall join themselves to thee as members, than the Jewish synagogue, which only seems to have God for her husband. This shall be as sure to thee as if thou hadst all these children about thy hand already.

“28 Now we, brethren, as Isaac was, are the children of promise.”

Ver. 28. Now, brethren, to apply the whole unto us, ye see, we who adhere to the covenant of grace are such children as Isaac, even children of promise; born unto God by virtue of the absolute promise; not by the strength of nature or by our own works.

“29 But as then be 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 bond-woman and her son: for the son of the bond-woman shall not be heir with the son of the free-woman.”

Ver. 29. But as then Ishmael persecuted Isaac, so now the legalists, children of the covenant of works, persecute the children of the covenant of grace. Ver. 30. Nevertheless, what saith the scripture? even, Cast out Hagar and Ishmael, for he shall not be heir with Isaac. The same sentence is passed against the Jewish synagogue, and all that adhere to the covenant of works; they shall be rejected and cast out of the society of saints, for they shall never partake of the heavenly inheritance with those that believe in Christ.

“31 So then, brethren, we are not children of the bond-woman, but of the free.”

Ver. 31. Therefore, seeing we are children of promise, as Isaac, we are not under the legal covenant, but the covenant of grace, and so are free; and therefore have nothing to do with the law.

—Thomas Boston, The Whole Works of Thomas Boston: Sermons and Discourses on Several Important Subjects in Divinity, ed. Samuel M‘Millan, vol. 6 (Aberdeen: George and Robert King, 1849), 266–68.