Chapter 21Chapter 21 records the birth of Isaac, the heir of the covenant. God gives Abraham and Sarah the promised son at the very time He had spoken, fulfilling His word. Isaac is born 25 years after Abraham was first called at the age of 75(verses 1–7). Isaac is circumcised on the eighth day and grows as the heir of God’s promised inheritance. However, Ishmael, who was born through Hagar, moc..