(12) In Isaac shall thy seed be called--Heb., in Isaac there shall be called to thee a seed: that is, the seed that shall especially be accounted thine, and which, as such, shall inherit the promises, will be that sprung from Isaac.

Verse 12. - And God said unto Abraham, - probably in a dream, or night vision (vide Ver. 14) - Let it not be grievous in thy sight because of the lad, and because of thy bondwoman; - who was never recognized by God as Abraham's wife (cf. Genesis 16:8) - in all that Sarah hath said unto thee, hearken unto her voice. Though Sarah's counsel was approved by God, it does not follow that her conduct was. On a former occasion Abraham's hearkening unto Sarah's voice had led to sin (Genesis 16:2); this time it would lie exactly in the line of duty. For in Isaac shall thy seed be called. Literally, in Isaac shall seed (i.e. posterity) be called to thee; meaning neither, "by Isaac shall thy seed be called, or named" (Hofmann, Kalisch, Ainsworth), nor, "in Isaac shall thy seed be called into existence" (Dreschler); but, "in Isaac shall there be posterity to thee which shall pass as such," i.e. be called or recognized as such (Keil); or, more simply, "in Isaac," i.e. in the line of Isaac, "shall be called to thee a seed," i.e. a seed par excellence, the seed already promised (Bleek, Delitzsch, Rosenmüller, Alford, Murphy).

21:9-13 Let us not overlook the manner in which this family matter instructs us not to rest in outward privileges, or in our own doings. And let us seek the blessings of the new covenant by faith in its Divine Surety. Ishmael's conduct was persecution, being done in profane contempt of the covenant and promise, and with malice against Isaac. God takes notice of what children say and do in their play; and will reckon with them, if they say or do amiss, though their parents do not. Mocking is a great sin, and very provoking to God. And the children of promise must expect to be mocked. Abraham was grieved that Ishmael should misbehave, and Sarah demand so severe a punishment. But God showed him that Isaac must be the father of the promised Seed; therefore, send Ishmael away, lest he corrupt the manners, or try to take the rights of Isaac. The covenant seed of Abraham must be a people by themselves, not mingled with those who were out of covenant: Sarah little thought of this; but God turned aright what she said.And God said unto Abraham,.... Either by an articulate voice, or by an impulse on his mind, suggesting to him what he should do, being no doubt in great perplexity how to conduct between his wife and his son, but God determines the case for him, and makes him easy:

let it not be grievous in thy sight because of the lad, and because of the bondwoman: that is, let not the motion displease thee, which Sarah has made, to turn out the bondwoman and her son; let not thine affection to the one and to the other hinder compliance with it; do not look upon it as an ill thing, or as an hard thing; it is but what is right and proper to be done, and leave the bondwoman and her son to me; I will take care of them, be under no concern for them and their welfare:

in all that Sarah hath said unto thee, hearken to her voice; the Targum of Jonathan adds, for she is a prophetess: and indeed in this affair she spoke under a spirit of prophecy, according to the will of God; at least what she said became a divine oracle, and is called the Scripture, Galatians 4:30; for the word "all" here must be restrained to what she had said concerning Hagar and Ishmael, and their ejection, and not to be extended to everything she had said, or should say to Abraham, to which he was always to be attentive: whereas on the other hand, it became her, as a wife, to hearken and be obedient to the voice of her husband: but in this particular Abraham is bid to listen to her, and do accordingly, for the following reason:

for in Isaac shall thy seed be called; he, and those that descended from him, should be called and reckoned the seed of Abraham more especially; and Abraham's seed in his life should inherit the land of Canaan, given to him and his seed for an inheritance: and this is a good reason why the bondwoman and her son should be cast out, that they or their offspring might not inherit the land with Isaac, or his descendants; and particularly from Abraham in his line, and not in the line of Ishmael, should the Messiah spring, that seed in whom all the nations of the earth should be blessed; and therefore a separation was necessary, that this might abundantly appear.