Why did God wait until Abraham and Sarah were old to give them a son? Why did he wait until the situation had become humanly impossible and only absolute faith in him could bring about the promise? It was because this seed was to be birthed completely in faith.
Abraham and Sarah both knew the amazing promise of God to give them a son but their faith was not yet ripe. Sarah became impatient and devised a plan and Abraham went along with it. Ishmael was born and while it was true that he was of Abraham’s seed, he was not the promised heir. In a twist of human nature, Sarah then raged at her husband with jealousy and blamed him. They were both trying to make God’s promise come about by their own efforts (see Genesis 16:1-15).
Perhaps you are wondering why your promise has not yet been fulfilled. You are trying to figure out how God will meet your need, becoming all bogged down concentrating on the hindrances instead of the possibilities.
Abraham and Sarah were given their miracle child but not until they became fully persuaded that God would fulfill his promise in his way and in his time. We know they became strong in their faith but only after having wavered — and Ishmael was a sign of that lapse in faith. Later, however, it was said of Abraham: “He did not waver at the promise of God through unbelief, but was strengthened in faith, giving glory to God, and being fully convinced that what He had promised He was also able to perform” (Romans 4:20-21).
Beloved, any wavering you have comes from unbelief; do not try to figure things out or debate the issues within yourself. Having faith means you have settled all your arguments and are fully persuaded that God will keep his Word. He wants to do the impossible because the more impossible the situation, the more glory he receives!