New Defender's Study Bible Notes
21:1 as he has said. Despite their age–Abraham was one hundred and Sarah was ninety (Genesis 17:17)–God miraculously fulfilled His promise, made twenty-five years earlier (Genesis 12:4,7), to give them a son. Furthermore, the promise was fulfilled at the “time appointed” He had promised (Genesis 18:14).
21:4 eight days old. The act of circumcision was not only the sign of the Abrahamic covenant (see note on Genesis 17:11), but was also a significant contribution to health, of both husband and wife, as modern medical knowledge confirms. It is also well established that, as far as the health of the infant is concerned, the eighth day is the optimum time for performing the operation.
21:7 his old age. When God heals miraculously, he does it instantly and completely. Sarah’s body was so rejuvenated that although she was ninety years old, she was able both to bear a child and even to nurse him. Abraham was so “young” again that even at one hundred years of age he could later father six more sons of Keturah many years later, after Sarah’s death.