Question: "How long did Abraham and Sarah have to wait for Isaac?"
Answer: Genesis 12 begins the story of Abraham (then called Abram) and his barren wife Sarah. Verses 1 through 4 record God’s first words to him about a homeland for his offspring. Even though the gift of a son is not directly mentioned in this first communication, God hinted at His plan for Abram. Abraham was 75 years old when he first received the promise, and Genesis 21:5 tells us he was 100 years old when Isaac was born. Sarah was 90. So Abraham and Sarah waited 25 years for the fulfillment of God’s promise.
In that 25 years between the time that Abram was promised a son and the birth of Isaac, Abram and Sarah had certain ideas of how they might facilitate the keeping of the promise. One was that Abraham’s steward, Eliezer, would become the heir of Abraham’s household (Genesis 15:2–3). Another idea was that Abraham could have an heir through a son conceived by Sarah’s slave, Hagar (Genesis 16:1–2). In both cases, God rejected those men as Abraham’s heirs, pointing Abraham and Sarah to a literal, miraculous fulfillment of the promise.
Abraham is called the father of faith (Romans 4:11–12) because of his response to God both in leaving his homeland and receiving a son in his old age. Genesis 15:4–5 again describes God’s promise to Abraham that his offspring would be as “the sands of the sea.” Even though Abraham was old and had no sons, he never doubted that God would do as He promised. He did not understand how such a thing could be possible, but he humbly accepted God’s word as truth. Mary had the same response when the angel Gabriel told her she would be the mother of the Messiah (Luke 1:26–38). She did not understand how such a thing could be possible since she was a virgin. But she never doubted that God would do as He said. That response is the kind of faith that pleases God (Hebrews 11:6).
Genesis 15:6 lays out the truth that salvation is by faith, apart from works: “Abram believed the Lord, and he credited it to him as righteousness.” Romans 4:3–5 and Galatians 3:5–7 elaborate on this truth. Just as Abraham was counted as righteous before he did anything worthy of praise, so we are counted as righteous by simply believing that the sacrifice of Jesus Christ is sufficient payment for our sin (2 Corinthians 5:21).
However, that faith produces actions that validate it. James 2:14–18 helps us understand the kind of faith Abraham had. It was a faith that acted. He moved because God said to move. He trusted because God said to trust. He prepared to welcome a son because God promised him a son. By acting on his faith, Abraham proved that he trusted God, and that trust was credited to his account. His faith in the promises of God saw him through years of waiting. He never doubted God’s goodness or His word, and, for that, God considered him righteous.