And Abraham was a hundred years old when his son Isaac was born to him.
Genesis 21:5 fixes a date on a miracle: the son God had pledged arrives when the father is a hundred. The number is not filler — it measures how long faith waited, and how far past natural hope God let things go before He acted.
Twenty-five Years of Waiting
The age here marks the end of a long wait, counted from the day Abraham left home. One writer traces the whole delay:
Through long and lonely years Sarah and he lived in this foreign land. They owned not so much as a foot of it. And there was no sign of the heir who would receive his flocks and his herds, his silver and his gold, and through whom God would fulfil His word... But not until eleven years after he entered the land was Ishmael born, and then not of Sarah. And the son of Hagar could not live before God. Abraham still had to wait... And though twenty-five years had passed since he left Haran, hope deferred did not make the heart sick.
God's Set Time
The chapter says the birth came at the time God had spoken of. Commentators press that phrase. The delay was not God being slow. He was keeping His own time, and nature could add nothing to it.
Nature can never do anything for God. The Lord must visit and the Lord must "do." Faith must wait, and nature must be still. Nature must be set aside completely, as a dead, worthless thing. Then the divine glory can shine out... There is such a thing as God's "set time," His "due season," and for this the faithful must be content to wait. The time may seem long, and hope deferred may make the heart sick. But the spiritual mind will always find relief in this: all of it is for the final display of God's glory... Nature had nothing to glory in. Man's extremity was God's opportunity. All is triumph when God is allowed to show Himself.
(Older translations read "at the set time.")
Life Out of Death
Abraham was a hundred, Sarah ninety. Children do not come at those ages. That is why the ages are recorded: this birth was not the work of nature.
What was impossible for man happened. Isaac was born, as we may say, on the principle of resurrection: a living child coming from parents who, as far as having children went, were dead.
The covenant runs through Isaac, not through Ishmael. Ishmael was born in the ordinary way, years earlier:
God's purpose, or secret, or covenant, rested on resurrection. It rested on the setting aside of the flesh in its strength and its resources. It rested on God doing something beyond and above nature... Ishmael may be blessed — and he is — but the covenant is with Isaac, and with Isaac alone. This tells us again, plainly, that God has taken resurrection as the principle of His action, the ground of His plans.
Faith Rewarded
The verse sits right before Sarah's words about laughter, and the two belong together. Her first laugh came from unbelief. This one does not.
Verse 5 tells us that Abraham was a hundred years old when Isaac was born. Faith did indeed have to wait, but was in no way disappointed: God is faithful... She had laughed at first when the Lord announced the set time for her to become a mother. When He challenged her about it, she added the shame of untruth. But grace changes everything here. God, she admits, made her laugh now. It was no longer within herself, but from Him; and others who heard would share her joy.
The Son of Promise Points to Christ
Writers read Isaac's impossible birth as a picture of a later one. Every natural hope had to run out first, so that the child came by promise alone.
Isaac is this child of promise. He was born to Sarah when she was 90 years old and Abraham 100. Their bodies were as good as "already dead" (Rom. 4:19), and any natural hope of children was long gone. But it was God who intervened in sovereign grace to fulfil His own word... Isaac then is a beautiful picture of Christ, the promised Seed (Gal. 3:16), who came to earth not because man kept the law successfully or anything like it, but because of the unbreakable promise of God, fulfilled by grace alone.
Summary
- Promise delayed — twenty-five years passed between the word and the child. God's "set time" is an appointment, not slowness.
- Promise held — waiting is not wasted; the whole delay works toward the display of God's glory.
- Promise, not flesh — at a hundred, nature is out of the running. Nature can never do anything for God, and the covenant runs through Isaac alone.
- Promise fulfilled — faith waited and was not disappointed. Grace turned Sarah's unbelieving laugh into shared joy.
- Promise embodied — a child born from bodies as good as dead begins the line that leads to the promised Seed, Christ.