Genesis 21:12, commentary and pictures

God told Abraham not to consider the matter grievous on account of the lad and the bondwoman but to listen to Sarah's voice and send Hagar and Ishmael away, refusing to let grief and love and affection for them hold him back. That sending away was the course required to restore peace and happiness to the household. In so directing him, God did not rebuke Sarah but upheld her rights as Abraham's first and only true wife and as Isaac's mother, rights that no other person was entitled to share. The instruction teaches that the marriage relation and the rights and happiness belonging to it must be respected and guarded and protected, even at great sacrifice and personal cost.

The verse fixes that Abraham's promised seed and descendants and line would be called and counted through Isaac, not Ishmael, and that the promise of a numberless seed rested on Isaac as the child miraculously given by God. Ishmael's separation did not mean God forsook him, for God would preserve him and make him a great nation. Abraham's marriage to Hagar had come from distrust and doubt of God's power to fulfill the promise through Sarah, a failure of faith shared by Abraham and Sarah. The instruction taught Abraham that affection must not prevent obedience to God.

This promise through Isaac sustained Abraham when he later faced the command to offer Isaac, for he trusted that the God who gave Isaac life could restore it by raising him from the dead. Because God had fixed the promise on Isaac, Abraham did not suppose when commanded to offer Isaac that the promise would instead be fulfilled through Ishmael.