Genesis 21:11-13

"And the thing was very grievous in Abraham's sight, on account of his son. And God said unto Abraham, Let it not be grievous in thy sight because of the lad, and because of thy handmaid; in all that Sarah saith unto thee, hearken unto her voice; for in Isaac shall thy seed be called. And also of the son of the handmaid will I make a nation, because he is thy seed."

Abraham and Sarah were then reaping the bitterness created by themselves when they chose to introduce a slave girl into Abraham's bedchamber as his wife. How far better it would have been if they had found the faith and strength to await the fulfillment of God's promise in His own good time! There can hardly be any doubt that Abraham would have avoided freeing the slave woman and sending her away had it not been for the direct command of God that he should "hearken to the voice" of Sarah. What a heart-wrenching decision it was, but Abraham courageously faced the problem and resolved it as God commanded him.

"He is thy seed ..." Willis and others frequently stress that "seed" in this passage is a collective noun in singular form, but with a plural meaning. "It is clear that the reference is to the Israelite people,"[7] or, as in the case of Ishmael, the whole people descended from him. However, such a simplistic view of this word is insufficient. There are no less than five definite meanings of this term in the Bible:

1. "In Isaac shall thy seed be called" (Genesis 21:12), is a reference to the select Hebrew line, through whom the Messiah would be born, and it did not include Ishmael.
   
2. "I will make (Ishmael) a nation, because he is thy seed," stands for the fleshly posterity of Abraham as distinguished from the line of Isaac.
   
3. Also, there are those among the true line of Isaac who were distinguished from the racial Jews of both the lines of Isaac and Ishmael, because they were persons of like faith and purpose of Abraham. In this sense, Zacchaeus was called by Jesus, "a son of Abraham," (Luke 19:9); but the Pharisees, of exactly the same racial extract, were called "sons of the devil" (John 8:44).
   
4. In the specific and ultimate sense, Christ is the "Seed Singular" of Abraham, being called THE SON OF ABRAHAM in the first verse of the N.T. That this meaning is the true one in certain O.T. passages is evident from Paul's words: "Now to Abraham were the promises spoken, and to his seed. He saith not, And to seeds, as of many; but as of one, and to thy seed, which is Christ" (Galatians 3:16).
   
5. But there is even a more general meaning, having no racial overtones whatever. "And if ye are Christ's, then are ye Abraham's seed, heirs according to the promise" (Galatians 3:29). Thus, a great deal more must be observed concerning the term "seed" than the mere fact of its being a collective noun and usually understood as plural.
   

God's promise to make Ishmael a nation was likely given as an encouragement for Abraham to carry out the divine instructions.

One other thing should be noticed here. Although wives in the N.T. are commanded to be "in subjection" to their own husbands, the example of Sarah who stood up against the wishes of her husband, is a rightful qualifier of that apostolic instruction. There are times when wives should indeed take things in their own hands despite the wishes of their husbands, and here is a glorious example of a beloved wife who did so. She is a type of "The Jerusalem which is above, which is our mother" (Galatians 4:25,26).