Charles Spurgeon Commentary Galatians 4:27-29

"For it is written, Rejoice, thou barren that bearest not; Break forth and cry, thou that travailest not: For more are the children of the desolate than of her that hath the husband. Now we, brethren, as Isaac was, are children of promise. But as then he that was born after the flesh persecuted him [that was born] after the Spirit, so also it is now." — Galatians 4:27-29 (ASV)

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, 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.

The child of Hagar could not abide the child of Sarah, and those who seek salvation by the works of the law, and by outward ceremonies, cannot endure the children of faith.