Genesis 12.4 notes that Abraham was seventy-five years old when he left Haran out of obedience to God to go to the land that God would show him. In 12.7, God promised Abraham that he would give this land to Abraham’s seed. Abraham had no heir; Sarah was barren. In 13.14-17, when Abraham’s and Lot’s camps quarrel over the land, God again promises to give the land to Abraham and his seed, and further promises that his seed would be innumerable. In chapter 15, Abraham is having doubts about having an heir. God reiterates: Abraham will have an heir and the land will belong to his descendents. About ten years after Abraham leaves Haran, when he is eighty-five (16.3), he acquiesces to his wife’s exhortation to make God’s promise happen by their own efforts, and he goes in to Hagar, Sarah’s slavegirl, to sire a son by her. That, of course, goes badly. Twelve years after Ishmael’s birth–Abraham is now ninety-nine (17.1)–God again promises Abraham an heir, and within the year, when Abraham is one hundred years old (21.5), Isaac is born.
Twenty-five years is a long time for a person to wait for a promise from God. And Abraham clearly struggles with doubt. When he gives in to his doubt, it has problematic consequences. Yet, he continues to struggle to maintain faith, and ultimately, he enjoys the gracious promises of God. And because of his faith, we enjoy those same promises as well. (Cf. Hebrews 11:8-19)