“Therefore [Sarah] said to Abraham, “Drive out this maid and her son, for the son of this maid shall not be an heir with my son Isaac.” The matter distressed Abraham greatly because of his son. But God said to Abraham, “Do not be distressed because of the lad and your maid; whatever Sarah tells you, listen to her, for through Isaac your descendants shall be named.” Gen. 21:10-12
We must remember that Abraham loved Ishmael just as he loved Isaac. It is all too easy to think that since Ishmael does not have any major significance to Christianity that we need not consider him much, but alas, Abraham is greatly distressed over Sarah’s desire to send him away.
Ishmael is no longer a mere child; Abraham has been a father to him for probably 16 years, if not longer by this time. (Ishmael is 13 years older than Isaac, and Isaac was recently weaned from the breast, so he was likely between 2-3 years old). In addition, until the birth of Isaac, Abraham was likely assuming that Ishmael was the son whom the promises of God would come.
Certainly Abraham does not want to send Hagar and Ishmael off into the desert, but in an interesting turn of events, God instructs Abraham to “do whatever Sarah tells you.” Contrast this with the words of God to Adam in Genesis 3:17: “Because you have listened to the voice of your wife…” which was followed by a curse. The difference is this: Eve’s actions were in direct disobedience to God’s will, whereas here Sarah is in line with God’s will – not in her anger, but rather in the intended goal: God wanted Isaac to be raised apart from Ishmael and Hagar.