Matthew Henry Commentary Genesis 21:1-8

"And Jehovah visited Sarah as he had said, and Jehovah did unto Sarah as he had spoken. And Sarah conceived, and bare Abraham a son in his old age, at the set time of which God had spoken to him. And Abraham called the name of his son that was born unto him, whom Sarah bare to him, Isaac. And Abraham circumcised his son Isaac when he was eight days old, as God had commanded him. And Abraham was a hundred years old, when his son Isaac was born unto him. And Sarah said, God hath made me to laugh. Every one that heareth will laugh with me. And she said, Who would have said unto Abraham, that Sarah should give children suck? For I have borne him a son in his old age. And the child grew, and was weaned. And Abraham made a great feast on the day that Isaac was weaned." — Genesis 21:1-8 (ASV)

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 whom 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 He sets, and that is the best time.

Isaac means “laughter,” and there was good reason for the name (Genesis 17:17; Genesis 18:13). When the Sun of comfort is risen upon the soul, it is good to remember how welcome the dawning of the day was. When Sarah received the promise, she laughed with distrust and doubt. When God gives us the mercies we began to despair of, we ought to remember with sorrow and shame our sinful distrust of His power and His promise when we were in pursuit of them.

This mercy filled Sarah with joy and wonder. God's favours to His covenant people are such as surpass their own and others' thoughts and expectations: who could imagine that He should do so much for those that deserve so little, even for those that deserve so ill? Who would have said that God should send His Son to die for us, His Spirit to make us holy, His angels to attend us? Who would have said that such great sins should be pardoned, such humble services accepted, and such worthless worms taken into covenant?

A short account of Isaac's infancy is given. God's blessing upon the nursing of children, and the preservation of them through the perils of the infant age, are to be acknowledged as remarkable instances of the care and tenderness of the Divine providence. See Psalm 22:9-10; Hosea 11:1–2.