Verse 5. - And Abraham was an hundred years old (cf. Genesis 17:1, 17), when his son Isaac was born unto him. Literally, at the time of bearing to him (ἐν τῷ τεκεῖν) Isaac (vide Gesenius, 'Gram.,' § 143). Thus Abraham had waited twenty-five years for the fulfillment of the promise - a remarkable instance of faith and patience (Romans 4:20), as Isaac's birth was a signal display of Divine power (Romans 4:17; Hebrews 11:12). Whether Isaac was born at Gerar or at Beersheba cannot with certitude be inferred.

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, ch. 17:17; 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 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, nay, 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 mean 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 signal instances of the care and tenderness of the Divine providence. See Ps 22:9,10; Ho 11:1,2.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 (o), 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 17:1. 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 (p), Isaac was born A. M. 2108, and before Christ 1896, and probably at Beersheba, see Genesis 21:33.

(o) Apud Euseb. Praepar. Evangel. l. 9. c. 21. p. 425. (p) Annales Vet. Test. p. 9.