Gen 17:19

“אבל VERILY — This word implies confirmation of a statement, just as (Genesis 42:21) “Verily (אבל) we are guilty”, and (2 Kings 4:14) “Verily (אבל) she has no son”. וקראת את שמו יצחק AND THOU SHALL CALL HIS NAME ISAAC — with reference to his laughter. Some say the name has reference to the ten (י) trials, the ninety (צ) years of Sarah’s age, the eight (ח) days of the circumcision and the hundred (ק) years of Abraham’s age (Genesis Rabbah 53:7). והקמתי את בריתי AND I WILL ESTABLISH MY COVENANT WITH HIM — Why is this stated? Is it not already written (Genesis 17:9) “as for thee, thou shalt keep My Covenant, thou and thy seed etc.”? But since is said, (Genesis 17:7) and I will establish My Covenant between Me and thee and thy seed after thee in their generations”, it might be inferred that the children of Ishmael and the children of Keturah (who are all descendants of Abraham) are included in this promise of the establishment of the Covenant, therefore it states here (Genesis 17:19) “And I will establish My Covenant with him (Isaac), not with the others” (cf. Sanhedrin 59b). If so, why is there a repetition of the statement that the covenant will be established with Isaac in the words (Genesis 17:21) ואת בריתי אקים את יצחק “but My Covenant will I establish with Isaac”? But it is repeated in order to add the words אשר תלד so as to teach that he was holy from his birth (Shabbat 137b). Another explanation of why 5:21 is repeated is: R. Aba said, “from here Scripture draws a logical conclusion regarding the son of the Mistress (Sarah) from what is said regarding the son of the handmaid (Hagar): It is written (Genesis 17:20) “Behold I have blessed him and will make him fruitful”, and this refers to Ishmael: Surely then, it follows logically that “I must establish my covenant (of blessing) with Isaac also!” (Genesis Rabbah 47:5). את בריתי [I WILL ESTABLISH] MY COVENANT — The Covenant of circumcision shall be transmitted only to the descendants of Isaac.”

“‘AVAL’ SARAH THY WIFE. This is like the verse, “Aval” she has no son, the word aval meaning “only.” He thus said: “Only the son of whom I informed you, your wife Sarah will give birth to, and with him will I establish My covenant forever and with his seed after him; and Ishmael I will bless by making his children numerous, but this will not be because of My covenant with him.””

“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, Gen 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 Gen 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.”