Galatians 4:26-31 But Jerusalem which is above is free, which is the mother of us all. For it is written, Rejoice, thou barren that bear not; break forth and cry, thou that travail not: for the desolate has many more children than she which has an husband. Now we, brethren, as Isaac was, are the children of promise. But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now. Nevertheless what sad 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. So then, brethren, we are not children of the bondwoman, but of the free.
‘Jerusalem which is above is free, which is the mother of us all.’ The New Jerusalem in heaven is a free city and the mother of us all (Heb. 11:10-16; 12:22-23; 13:14; Rev. 3:12; 21:1-22:5). In other words, Jerusalem is the place where God lives and who adopts us into His family (4:26-27).
‘Rejoice, thou barren that bear not; break forth and cry, thou that travail not: for the desolate has many more children than she which hath an husband.’ This verse predicts the salvation of the Gentiles during the rejection and dispersion of Israel. Paul quoted this passage to prove that more men will be saved under the new covenant than under the old one, which is cast out and abolished. Quoted from Isaiah 54:1.
‘Now we, brethren, as Isaac was, are the children of promise.’ We who are Christians are the children of the promise and the new covenant like Isaac and are being persecuted as he was (4:28-29).
‘Born after the Spirit, even so it is now.’ Men were born again in Old Testament days (4:29; Deut. 32:18; Ps. 19:7; 32:1-2; 51:10-13; 103:3; Isa. 1:16-18). This was experienced by having faith in the coming redemption on the cross, whereas we look back to the cross.
‘Cast out the bondwoman and her son.’ Quoted from Genesis 21:10. This represents the old covenant being cast out as having no part in the Christian program. Christians are not under one detail of the law of Moses, including the ten commandments. When you truly love you will never transgress the moral law (Rom. 13:8, 10). Deprive any man of his wife (Rom. 13:9). Rob any woman of her virtue. Take the life of anyone. Hate anyone (1Jn. 3:15). Take the property of anyone. Rob a man of his good name. Covet one thing of another. Love only yourself. Work ill toward anyone (Rom. 13:10).
‘For the son of the bondwoman shall not be heir with the son of the freewoman.’ Just as Hagar and Ishmael were cast out of Abraham’s home and received no inheritance with Isaac, so the law of Moses and the gospel cannot mix. They are two different contracts. One is abolished and the other is still in force (4:30-31).