Genesis 21:12

And God said unto Abraham, Let it not be grievous in thy sight because of the lad and because of thy bondwoman; in all that Sarah hath said unto thee hearken unto her voice; for in Isaac shall thy seed be called. It was God who decided the matter, and in Sarah's favor. Abraham was to set aside personal feelings and considerations, both as to Ishmael and as to Hagar, and heed the voice of his wife. For Isaac was to be the bearer of the Messianic promise; through his lineage the blessing of the Lord was to come upon the nations, in the person of the Messiah. Cf Romans 9:7; Hebrews 11:18. The words of Sarah have a special spiritual meaning, as St. Paul shows, Galatians 4:29: "As then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now. " Those that are spiritual-minded, the children of God, are derided and persecuted by the carnal-minded, by the children of the world. The will of the Lord is that His children should separate themselves in all spiritual matters from the children of the world, and avoid all the pitfalls which are placed for the unwary feet of such as seek the friendship of His enemies.