His son by the slave woman was born according to the flesh, but his son by the free woman was born as the result of a divine promise.
References for Galatians 4:23
These things are being taken figuratively: The women represent two covenants. One covenant is from Mount Sinai and bears children who are to be slaves: This is Hagar.
Now Hagar stands for Mount Sinai in Arabia and corresponds to the present city of Jerusalem, because she is in slavery with her children.
But the Jerusalem that is above is free, and she is our mother.
References for Galatians 4:26
For it is written: “Be glad, barren woman, you who never bore a child; shout for joy and cry aloud, you who were never in labor; because more are the children of the desolate woman than of her who has a husband.”
References for Galatians 4:27
Now you, brothers and sisters, like Isaac, are children of promise.
References for Galatians 4:28
At that time the son born according to the flesh persecuted the son born by the power of the Spirit. It is the same now.
References for Galatians 4:29
But what does Scripture say? “Get rid of the slave woman and her son, for the slave woman’s son will never share in the inheritance with the free woman’s son.”
References for Galatians 4:30
Therefore, brothers and sisters, we are not children of the slave woman, but of the free woman.
References for Galatians 4:31