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.
Note 10 at Ga 4:30: Here, Paul was taking the Galatians back to the Scriptures. He was speaking from Ge 21:9-14. The point was that just as Hagar and Ishmael would not have any part in the inheritance of Isaac, so those of the covenant of Law with its legalism will not inherit the promise of justification that comes by faith.