This is well-known history to a Jew and to many of us. Abraham first had a son by a "bondwoman", that is, a servant or slave. We know her as Hagar. Then he had a son by a freewoman, namely his own wife Sarah (4:22).

He who was of the bondwoman, namely Ishmael, was born according to the flesh. Now, birth is a flesh thing. Everyone must have a flesh and blood birth. Everyone is born according to the “flesh” in that respect.

But the means to that birth, though also of flesh, can be guided by men or by God. People have babies every day, many out of wedlock, many with a partner God never intended, and so on. Ishmael was of this nature, and because Hagar, his mother, was not the legal wife of Abraham, and because the plan to have a child through her was conceived in the carnal mind, he is said to be born carnally, or in the flesh (4:23).

On the other hand, Isaac, who also came into the world by the union of a man and a woman, a real flesh and blood child, came in through a legal and God-blessed, even God-promised union. He was therefore a child of promise. God said Abraham would have a son from his own body and his wife's. There is no other but Isaac who was born this way, by promise, by power of God, yet fully human in nature.

Being born out of wedlock is not unpardonable or to be judged. This is not the point of the story here. He uses Isaac and Ishmael to make a point, symbolically.

What do the two covenants stand for?

4:24-27 24These things serve as illustrations, for the women represent two covenants. One covenant is from Mount Sinai and bears children into slavery: This is Hagar. 25Now Hagar stands for Mount Sinai in Arabia and corresponds to the present-day Jerusalem, because she is in slavery with her children. 26But the Jerusalem above is free, and she is our mother. 27For it is written: “Rejoice, O barren woman,

who bears no children;

break forth and cry aloud,

you who have never travailed;

because more are the children of the desolate woman

than of her who has a husband.”

He says that these two sons can stand for the two covenants, namely the covenant of Mt Sinai, and the New covenant of the Spirit.

Ishmael, born from Hagar the servant, born of the carnal will of man, matches up with the Law, born from Mt. Sinai (4:24).