GENESIS CHAPTER 21
The purpose of the observation stage is to maintain focus on the text at hand within the normative rules of language, context and logic which limits the observer to the content offered by the book of Genesis. This will serve to avoid going on unnecessary tangents elsewhere; and more importantly, it will provide the framework for a proper and objective comparison with passages located elsewhere in Scripture.
Remember that something elsewhere may be true, but in the text at hand it may not be in view.
I) [Gen 21:1-12]:
(v. 21:1 ASV) "And Jehovah hath looked after Sarah as He hath said, and Jehovah doth to Sarah as He hath spoken;
(v. 21:2 ASV) and Sarah conceiveth, and beareth a son to Abraham, to his old age, at the appointed time that God hath spoken of with him;
(v. 21:3 ASV) and Abraham calleth the name of his son who is born to him, whom Sarah hath born to him -- Isaac;
(v. 21:4 ASV) and Abraham circumciseth Isaac his son, [being] a son of eight days, as God hath commanded him.
(v. 21:5 ASV) And Abraham [is] a son of a hundred years in Isaac his son being born to him,
(v. 21:6 ASV) and Sarah saith, 'God hath made laughter for me; every one who is hearing laugheth for me.'
(v. 21:7 ASV) She saith also, 'Who hath said to Abraham, Sarah hath suckled sons, that I have born a son for his old age?'
(v. 21:8 ASV) And the lad groweth, and is weaned, and Abraham maketh a great banquet in the day of Isaac's being weaned;
(v. 21:9 ASV) and Sarah seeth the son of Hagar the Egyptian, whom she hath borne to Abraham, mocking,
(v. 21:10 YLT) and she saith to Abraham, 'Cast out this handmaid and her son; for the son of this handmaid hath no possession with my son - with Isaac.'
(v. 21:11 NKJV) And the matter was very displeasing in Abraham's sight because of his son.
(v. 21:12 NKJV) But God said to Abraham, "Do not let it be displeasing in your sight because of the lad or because of your bondwoman. Whatever Sarah has said to you, listen to her voice; for in Isaac your seed shall be called."
God tells Abraham not to consider it wrong, (lit., distress), concerning the expulsion from his household of his son, Ishmael and Ishmael's mother, Sarah's female slave, (Hagar). God tells Abraham to heed the voice of Sarah, Abraham's wife; who has demanded that Abraham cast out the female slave and her son from their household, (cf. Gen 21:10). The reason for God's command is that it is in Isaac that Abraham's seed will be called relative to God's promise of eternal life and that being fulfilled in His provision of a son by faith not by blood relations. It is evident that God wants to make it clear that His promises to Abraham and all mankind were solely through His sovereignty - through Abraham's Seed via relations with his wife Sarah alone, not through anyone else. This excludes Hagar and Ishmael her son with Abraham, for Ishmael was Abraham and Sarah's solution to their childlessness, not God's. There can be no changes in what God has promised, nor risk of confusion, nor any modification by man of what God has promised.
Although Ishmael was born out of Abraham's relations with Hagar, and therefore one cannot say that Ishmael being a physical descendant of Abraham hence fulfills God's promise of a son through whom will be a Messiah-Savior. God's statement, "for in Isaac your seed shall be called [Abraham]" refers to God's promise through Abraham and his son Isaac with Sarah, not through anyone else. A promise that has spiritual implications is in view and is relative to faith in God's promise to Abraham of eternal life through a son by Abraham's wife Sarah, not Hagar.
Earlier passages in Scripture indicate that God formally made a unilateral covenant with Abraham when Abraham believed in God's promise that through his seed would come a Messiah-Savior Who would make provision for eternal life for Abraham and for all the families of the earth throughout all the ages. When Abraham believed in that promise, God declared Abraham righteous unto eternal life, (cf. Gen 15:6). At that time, God ratified and confirmed His covenant with Abraham unilaterally so that all those who expressed a moment of faith in God's promise of a Messiah-Savior through the Seed of Abraham would be declared righteous unto eternal life. Abraham was an example for all individuals of all ages.
Hence it is established that through faith in God's provision through the seed of Abraham, Isaac and through the Seed of Isaac, (Jesus Christ), would come justification unto eternal life - not through Ishmael or any son of Abraham. Notice that anyone born through Ishmael would in the physical sense be of the seed of Abraham, but not in the sense of the promises of God to Abraham through Isaac and all the families of the earth, which is a spiritual promise through the specific physical descendancy of Abraham through Isaac. In this case, God was refering to the seed of His promise of eternal life which decidedly comes only through Isaac and only by faith not by blood relations.