3 And Abraham gave to his son, to whom Sarah had given birth, the name Isaac. 4 And when his son Isaac was eight days old, Abraham made him undergo circumcision, as God had said to him. 5 Now Abraham was a hundred years old when the birth of Isaac took place.
Matthew Henry's Commentary on Genesis 21:3-5
Commentary on Genesis 21:1-8
(Read Genesis 21:1-8)
Few under the Old Testament were brought into the world with such expectations as Isaac. He was in this a type of Christ, that Seed which the holy God so long promised, and holy men so long expected. He was born according to the promise, at the set time of which God had spoken. God's promised mercies will certainly come at the time which He sets, and that is the best time. Isaac means "laughter," and there was good reason for the name, Psalm 22:9,10; Hosea 11:1,2.