And God said, Sarah thy wife shall bear thee a son indeed,.... This is repeated for the confirmation of it, and thus expressed to remove all doubt about it, if any there were, that hung upon Abraham's mind; as well as to let him know that the promise of a son by Sarah was not to be superseded by his prayer for Ishmael, for whom he might have a greater flow of natural affection than for his unborn son, in whom his seed should be called:

and thou shall call his name Isaac; which signifies "laughter"; and which name was given him from the laughter of Abraham at the promise of him, and not from the laughter of Sarah, which as yet was not; wherefore Josephus (p) is wrong when he suggests, that Isaac had this name from Sarah's laughing at God's saying, that she should bear a son: though his birth was matter of laughter and joy to both, as it was to all good people that heard of it, Genesis 21:8. So Polyhistor (q) from Melo, an Heathen writer, speaking of Abraham, says, that of his married or lawful wife one son was born to him, whose name in Greek is "Gelos", that is, laughter. Isaac is one of those the Jews (r) observe had his name given him before he was born; see Gill on Genesis 16:11,

and I will establish my covenant with him, for an everlasting covenant, and with his seed after him; the covenant of circumcision just made with Abraham, the promise of the land of Canaan to him and his posterity, and of the Messiah that should spring from him, until whose coming this covenant would continue, and therefore called everlasting.

(p) Antiqu. l. 1. c. 12. sect. 2.((q) Apud Euseb. Evangel. Praepar. l. 9. c. 19. p. 421. (r) Pirke Eliezer, c. 32. Shalshalet Hakabala, fol. 2. 1.

And God said, Sarah thy wife shall bear thee a son indeed; and thou shalt call his name Isaac: and I will establish my covenant with him for an {g} everlasting covenant, and with his seed after him.

(g) The everlasting covenant is made with the children of the Spirit. A temporary promise is made with the children of the flesh, as was promised to Ishmael.

19, 20. The blessings of the covenant are reserved for Isaac, but common blessings were abundantly promised to Ishmael; and though the visible Church did not descend from his family, yet personally he might, and it is to be hoped did, enjoy its benefits.

17:15-22 Here is the promise made to Abraham of a son by Sarai, in whom the promise made to him should be fulfilled. The assurance of this promise was the change of Sarai's name into Sarah. Sarai signifies my princess, as if her honour were confined to one family only; Sarah signifies a princess. The more favours God confers upon us, the more low we should be in our own eyes. Abraham showed great joy; he laughed, it was a laughter of delight, not of distrust. Now it was that Abraham rejoiced to see Christ's day; now he saw it and was glad, Joh 8:56. Abraham, dreading lest Ishmael should be abandoned and forsaken of God, put up a petition on his behalf. God gives us leave in prayer to be particular in making known our requests. Whatever is our care and fear, should be spread before God in prayer. It is the duty of parents to pray for their children, and the great thing we should desire is, that they may be kept in covenant with Him, and may have grace to walk before him in uprightness. Common blessings are secured to Ishmael. Outward good things are often given to those children of godly parents who are born after the flesh, for their parents' sake. Covenant blessings are reserved for Isaac, and appropriated to him.