9 ¶ And Sarah saw the son of Hagar the Egyptian, which she had born unto Abraham, mocking. 10 Wherefore she said unto Abraham, Cast out this bondwoman and her son: for the son of this bondwoman shall not be heir with my son, even with Isaac. 11 And the thing was very grievous in Abraham’s sight because of his son. 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. 13 And also of the son of the bondwoman will I make a nation, because he is thy seed.
I mentioned the rabbinic opinion that the four hundred years of bondage began with this mocking back in Genesis 15:12-16. Sarah does not want Ishmael, who is not her offspring, to share the inheritance with Isaac. Sarah, to Abraham’s dismay, wants him to send Hagar and his son, Ishmael, away. God confirms that Isaac will be the vehicle through which He performs the covenant. He tells Abraham to listen to Sarah and God will make a nation out of Ishmael because he, too, is Abraham’s son. It is just that he is the son of Abraham and Sarah’s presumption, not the son of God’s promise.
Paul uses this in his argument in regard to comparing the Law to Grace. He likens those under Grace to be like Isaac, son of a free woman, while those under the Law to be like Ishmael, son of a slave. The Law is bondage, Grace is freedom.
Galatians 4:21 ¶ Tell me, ye that desire to be under the law, do ye not hear the law? 22 For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman. 23 But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise. 24 Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar [Hagar] . 25 For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children. 26 But Jerusalem which is above is free, which is the mother of us all. 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. 28 Now we, brethren, as Isaac was, are the children of promise. 29 But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now. 30 Nevertheless what saith 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. 31 So then, brethren, we are not children of the bondwoman, but of the free.
So, why do so many fundamentalist preachers keep trying to put their congregations back under the Law?