Summary: Abraham and Sarah try to help God keep His promise.
Genesis is a book about God’s eternal plan to redeem His children from their slavery to sin and death. Adam fell, but God promised a Son who would defeat the serpent who had tricked him. Every chapter of the Bible is an account of how His promise is fulfilled, and Genesis 16 is no different.
Abraham was an idol worshipping Gentile, but God called him to a Promised Land where he would be blessed. He obeyed but had no son and feared his servant would inherit the promise. So, God appeared again and promised not only a son but innumerable descendants who would possess the land. Abraham believed God, and to establish the covenant formally, God walked the blood path to make the promise a certain guarantee.
But a lot of time passes, and Abraham (who is 85 years old at this point) and his wife, Sarah, aren’t getting any younger. They slip into a sort of panic as the batteries on their biological clocks are quickly going dead. It’s in this panic that Sarah forms a plan to help God accomplish His purposes; but her plan backfires and a son is born who is not according to the promise. In Galatians Paul tells us the meaning of this story, so I want to start there: Tell me, ye that desire to be under the law, do ye not hear the law? 22For it is written, that Abraham had two sons, the one by a bondmaid, the other by a freewoman. 23But he who was of the bondwoman was born after the flesh; but he of the freewoman was by promise. 24Which things are an allegory: for these are the two covenants; the one from the mount Sinai, which gendereth to bondage, which is Agar. 25For this Agar is mount Sinai in Arabia, and answereth to Jerusalem which now is, and is in bondage with her children. 26But Jerusalem which is above is free, which is the mother of us all. 27For 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. 28Now we, brethren, as Isaac was, are the children of promise. 29But as then he that was born after the flesh persecuted him that was born after the Spirit, even so it is now. 30Nevertheless 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. 31So then, brethren, we are not children of the bondwoman, but of the free (Gal. 4:21-31).
Hagar and Ishmael are not heirs of the promise: “cast out the bondwoman and her son for the son of the bondwoman shall not be heir with the son of the free woman.” Later (Gen. 17:18-21) Abraham asks God to make His covenant with Ishmael instead of giving him another son, but God refuses saying, “I will establish my covenant with Isaac.” The whole point of this story we’re about to read (according to the Apostle Paul in the New Testament) is to make a distinction between the children of the promise and the children of the flesh. These two women and their descendants are allegories or symbols of God’s choice.