John Gill Commentary Genesis 21

And the Lord visited Sarah as he had said To Abraham, (Genesis 17:16Genesis 17:19Genesis 17:21) (Genesis 18:10Genesis 18:14); in a way of mercy and kindness, by fulfilling his promise, giving strength to conceive and bear a child; see (1 Samuel 2:21).

and the Lord did unto Sarah as he had spoken; which intends the same thing in different words; and the repetition is made to cause attention to God's fulfilment of his promise, who is always faithful to his word, even in things very difficult and seemingly impossible, as in the present case: hence the Targums of Jonathan and Jerusalem paraphrase it, God did a wonder or wonders for Sarah in causing her to conceive when she was so old, and in such circumstances as she was.

For Sarah conceived
This explains what is meant by the Lord's visiting her, and doing to her according to his word, see (Hebrews 11:11) ;

and bore Abraham a son in his old age ;
which circumstance is remarked, that the favour might appear the greater, and the more wonderful; or, "unto", or "for his old age" ^F14 , for the comfort of him in his old age, who having lived so many years under the promise of a son, and in the expectation of one, even of the promised seed, from whom the Messiah should spring, now has one, than which nothing could yield him greater consolation:

at the set time of which God had spoken to him ,
(Genesis 17:21) (18:10-14) ; God was not only faithful in fulfilling his promise, but in keeping the exact time of it.

FOOTNOTES:

• F14: (wynqzl) "pro senectute ejus", Vatablus; "senectuti ejus", Junius & Tremellius.

And Abraham called the name of his son that was born to
him, whom Sarah bore to him, Isaac .
] This is the name he was directed to give him, (Genesis 17:19) ; and he remembers the order, and is obedient to it; the reason of which name, which signifies laughter, was because of his laughing for joy at the promise made him, and there might be afterward a further reason for it, from Sarah's laughing because of distrust; and it might presignify the joy and laughter that would be expressed by others at his birth; and perhaps that he would be the object of the laughter and derision of his brother; so many events agreeing with his name.

And Abraham circumcised his son Isaac, being eight days old as ,
God had commanded him .
] (Genesis 17:12) , and was the first that we read of that was circumcised on that day, according to the divine precept.

which Abraham strictly observed; for though this was the son of the promise, and of his old age, for whom he had the most affectionate regard, yet he administered this bloody ordinance on him; nor did he defer it beyond the time, and was himself the operator, as it seems; all which shows his strict regard, and ready and cheerful obedience to the command of God.

And Abraham was an hundred years old when son Isaac was born
unto him .
] So that this was years after his departure from Haran, and coming into the land of Canaan, for then he was seventy five years of age, (Genesis 12:4) ; and this exactly agrees with the account of Demetrius, as related by Polyhistor, an Heathen writer ^F15 , who makes Isaac to be born just twenty five years from Abraham's coming into the land of Canaan, and who must be now an hundred years old, being ninety nine at the time the Lord appeared unto him, and promised him a son at the set time the next year, (Genesis 17:1Genesis 17:21) .

This is observed, both to show the wonderful favour to Abraham, and the faithfulness of God in the exact performance of his promise: according to Bishop Usher ^F16 , Isaac was born A. M. 2108, and before Christ 1896, and probably at Beersheba, see (Genesis 21:33Genesis 21:34) .

FOOTNOTES:

• F15: Apud Euseb. Praepar. Evangel. l. 9. c. 21. p. 425.
• F16: Annales Vet. Test. p. 9.