“In other words, it is not the natural children who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring”
Romans 9:8 NIV
Sarah was old and past the age of childbearing. God gave a promise to Abraham that she would bear a child. Abraham believed, God worked the miracle.
Isaac came not by Abraham’s works but as a fulfilment of God’s promise. Ishmael, on the other hand, came purely by natural efforts; he was born by Hagar, Sarah’s maid.
But guess what? God said to Abraham: “…In Isaac shall thy seed be called”
The true child of Abraham was the child of promise and not the child of works.
Paul calls the whole story an allegory. Speaking of Abraham he says:
“…His son by the slave woman was born in the ordinary way; but his son by the free woman was born as the result of a promise. These things may be taken figuratively, for the women represent two covenants. One covenant is from Mount Sinai and bears children who are to be slaves: This is Hagar”
Galatians 4:22-24 NIV
An allegory is a representation of an abstract or spiritual meaning through a concrete or material form. In other words, what is true about the physical lineage of the children of Abraham is true about his spiritual lineage.
The children of Abraham are not people merely born into his family. The children of Abraham are a result of faith in God and not works.
It is noteworthy that, not only Sarah but Rebecca and Rachel also couldn’t have children except by God’s miraculous intervention.
God made this happen in the physical to show us something spiritual. Abraham’s children would also be reckoned as a result of God’s promise to save mankind through Christ, and not as a result of man’s efforts in the law.
So we conclude thus: “And if ye be Christ’s, then are ye Abraham’s seed, and heirs according to the promise” (Galatians 3:29). Praise the Lord!