Galatians 4:27

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.

Paul here, by the inspiration of God, gives a figurative or allegorical explanation of the story of Hagar and Sarah. In addition to the historical truth of the story of Ishmael and Isaac he finds here a spiritual truth which typifies the permanent relation between those that are under the Law and those that are under the promise. For these two women, he says, are two covenants; they represent the two religions, that of the Law and that of the Gospel. This distinction holds true for all times. For although there are many races and nations in the world, they can still be divided into but two parties, namely, such as attempt to be justified before God by their own works and merits, and such as place their trust in the merits and righteousness of Jesus Christ alone. The first class is represented by the bond-woman Hagar, the covenant which originates from, that is, on, Mount Sinai, and bears children unto bondage; for every one that still hopes to earn salvation by the works of the Law is a spiritual offspring of Hagar, the slave, and is by virtue of that fact in bondage.

The apostle continues his explanation: For Mount Sinai is in Arabia, The mountain on which the Law was given is in the same country which became the home of Hagar's descendants, those that were children of bondage. And there is a further likeness in the fact that Hagar, as a bondswoman and the mother of a race in bondage, corresponded to, was in the same category with, the city of Jerusalem as it was when Paul wrote. Jerusalem was the capital of the Jewish race, if not of the Jewish nation; there dwelt the leaders of the people that continued to teach the necessity of keeping the Law in order to obtain salvation. As Hagar was thus in the state of bondage, so Jerusalem, the Jewish race, all that believed in the way of works and merits, are in the state of slavery, of spiritual bondage, knowing nothing of the liberty of the sons of God.

What a wonderful contrast, then: But the Jerusalem which is above is free, she is our mother. The apostle here speaks of the spiritual Jerusalem, or Zion, of the Church of Jesus Christ, that is, the total number of all believers that are scattered throughout the earth, who have the same Gospel, the same faith in Christ, the same Holy Spirit, and the same Sacraments, as Luther writes. This Church is the true mother of all believers; in her they have been regenerated unto new life, by her they are daily nourished; for God has given to His Church on earth the means of grace, to be used by all believers and dispensed to them day by day. In support of this apparently daring explanation the apostle cites Isaiah 54:1: Rejoice, thou barren one that bearest not; break forth and shout, thou that travailest not; for far more are the children of the desolate woman than of her that has a husband. This is a prophetic promise given to the Church of the New Covenant, to be fulfilled in the time of the Messiah. The contrast brought out by the prophet is that between the church of the Law, which was fertile and had many children, that is, that had the opinion that she alone was the real bride of the Lord, that her children alone were God's peculiar people, and the Church of the Gospel, of the evangelical promise, which, as the true bride of Christ, has brought forth a great number of descendants of every people, nation, and tongue; that is, the communion of believers and saints. It is a prophecy which will remain in power until the end of time; for so long the Gospel will be preached, by the power of which people are born in a spiritual manner.