Promise.
A promise of blessing, from Genesis 22:15-18 (NIV):
The angel of the LORD called to Abraham from heaven a second time and said, "I swear by myself, declares the LORD, that because you have done this and have not withheld your son, your only son, I will surely bless you and make your descendants as numerous as the stars in the sky and as the sand on the seashore. Your descendants will take possession of the cities of their enemies, and through your offspring all nations on earth will be blessed, because you have obeyed me."
God promised Abraham that he would be the father of a great nation, and it is through this bloodline that the rest of our story unfolds.