Thirteen years after Ishmael's birth, God appeared to Abram.  This was one of the seven revelatory encounters Abram had with God (see section 2.5).  God made a solemn covenant with Abram, and told him all the blessings that he would receive.  Almost the entire chapter 17 contains God's covenant promises:  Abram's and Sarai's names will be changed, nations and kings will come from them, they will have a homeland, God will be the God of their descendants.  He also established circumcision as a sign of inclusion in the covenant.  This also applied to foreigners who were bought as slaves.  Lastly, He said that Sarah would bear Abraham a son.

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Abraham's initial response was incredulity:

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          Abraham fell facedown; he laughed and said to himself, "Will a son be born to a man a hundred years old? Will Sarah bear a child at the age of ninety?" (Gen 17:17)

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He then pleaded for God to show favor to Ishmael.  This showed that he loved Ishmael, and had planned for him to be his heir.  God replied by speaking about Ishmael far more favorably than he had to Hagar in the desert:

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           "I will surely bless him; I will make him fruitful and will greatly increase his numbers. He will be the father of twelve rulers, and I will make him into a great nation. But my covenant I will establish with Isaac" (Gen 17:20-21).
       
After God left him, Abraham and all males in his household were circumcised as a sign of fidelity to and inclusion in the covenant.  Yet Ishmael, despite being circumcised, was not going to share in the promise of God. 

          

Here is God's Yes and No comingled.  There was a limit to His blessing outside of the covenant, there was a boundary which Ishmael could not pass, due to the sovereignty of God.  It was not a matter of human choice or decision. It was not even a matter of moral goodness.  Ishmael's rejection was not due to any sin of his.  It was rather that God would not bless the cleverness of Sarai in "solving" the problem of childlessness.  God was not going to acknowledge her plan, nor the fleshly seed of Abram invested in her scheme.  Isaac was God's own project, accomplished in His time and in His way.  It moved to its own logic, a logic that baffled man: witness the reactions of both Abram and Sarah.

           

Like Hannah, the mother of Samuel, later, God was going to vindicate Sarah at the deepest level of her identity, at the very place she had failed and been humiliated, and at the time when she had given up all hope and  knew that she would go to the grave barren.  Sarah was not a particularly good woman, she was not like Mary the mother of Jesus.  Scripture shows her as somewhat shrewish, skeptical and vindictive (Gen 18:12-15, Gen 21:10).  But her fulfilment, the enshrinement of her name in all eternity as the mother of the Promisebearer, was not God's main intent.  He nowhere acknowledged her vindication.  She was just the vessel of His purpose and plan, and the rehabilitation of her reputation was an incidental byproduct of His main purpose  -- to bless all nations through Abraham, and to draw men's attention to His Presence in history.  The tragedy of this story is that, in spite of the great mercy shown to her by God, and the miracle wrought in her body, she could not extend grace, love and forgiveness to her own servant Hagar and her son Ishmael.