Paul is creating an allegory. He compares the slavery of those who follow the law with the freedom of those who come to God by faith in Christ.

“Be glad, barren woman, you who never bore a child; shout for joy and cry aloud, you who were never in labor; because more are the children of the desolate ...

For it is written: "Rejoice, O barren woman who does not bear children; break forth and shout, you who have no birth pains, because the children of the desolate ...

The desolate and barren woman who has no husband, has greater blessings from the Lord than the woman who has a husband!

For it is written, “Rejoice, O barren one who does not bear; break forth and cry aloud, you who are not in labor! For the children of the desolate.

27 For it is written: Rejoice, O barren woman who does not give birth. Break forth and shout, you who are not in labor, for the children of the desolate are ...

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.

For it is written: “Be glad, barren woman, you who never bore a child; shout for joy and cry aloud, you who were never in labor; because more are the children ...

The law was the tutor unto the time of Christ. Once Christ is come, the promise fulfilled. Now, by faith we are redeemed from the law that we might be adopted ...