To build upon what nickybr38 stated, God promised Abraham that his descendants would be more numerous than the stars in the sky. Abraham and Sarah laughed however, and didn’t trust God, didn’t believe that his promise was trustworthy or that God would do what he said. So Abraham and Sarah waited a while and when it didn’t happen, they devised their own plan and decided to have a child through Hagar.
Don’t we do the same thing? Don’t we sometimes have a “Plan B” just in case God doesn’t come through for us in the way we want?
It was really Sarah’s idea to have a child through Hagar and her plan backfired on her, brought her nothing but grief. It wasn’t until Sarah sends Hagar and Ishmael away that God starts to act in their lives, eventually making Sarah pregnant in Abraham’s 99th year and giving him a child when he is 100 years old.
Now Abraham trusts God. Abraham trusts God’s promise to make his descendants more numerous than the sand in the seashore that when God tells him to sacrifice Isaac, he still believes and recons that God can raise people from the dead.
It’s a story of trust. Adam didn’t trust God and for the first time in history, man is learning to trust God, to trust in his promises.
God’s promise was fulfilled in Jesus Christ, yet we still don’t trust.
-Tim-