Galatians 4-5
Quote From Eugene Peterson
The interrelated births of Isaac and Ishmael are treated carefully and meditatively in Genesis.  Out of the multiple meanings in the stories of the two sons, Paul selects a single truth and uses it to drive home his message on freedom: one son was born because God promised, the other son was born because Abraham and Sarah doubted.  Ishmael was a product of human impatience, the human trying to do god’s work for him; Isaac was the result of God doing his own work in his own time.  Ishmael caused nothing but trouble; Isaac continued in the faithful covenant of the freely loving God.  The great disaster of Abraham’s life was that he used Hagar to get what he thought God wanted for him; the great achievement of this life was what God did for him apart from any programs or plans that he put into action.
The lesson of that old piece of history is clear enough: the moment we begin manipulating lives in order to get control of circumstances, we become enslaved in our own plans, tangled up in our own red tape, and have tho live with grievous, unintended consequences — Ishmael’s descendants complicated the life of faith enormously for centuries.  The life of freedom is a life of receiving, of believing, of accepting, of hoping. Because God freely keeps his promises, we are free to trust.
Traveling Light - Modern Meditations on St. Paul’s Letter of Fredom by Eugene H. Peterson - P 131-132