For the son of the free woman to be the result of promise means that the birth of Isaac was miraculous.1 Given the age of Sarah, there was no way for her to bear a child unless the Lord intervened in a miraculous manner to give her strength, open her womb, and allow her to conceive. The Lord had promised Abraham that he would do so (Genesis 17:19; Genesis 18:10),2 and he acted to keep his promise.
23 But the son of the slave was born according to the flesh, while the son of the free woman was born through promise.