Genesis 17:20-21

"And as for Ishmael, I have heard thee: behold I have blessed him, and will make him fruitful, and will multiply him exceedingly; twelve princes shall he beget, and I will make him a great nation. But my covenant will I establish with Isaac, whom Sarah shall bear unto thee at this set time in the next year."

"Princes shall he beget ..." Note that the superiority of Isaac over Ishmael is doubly apparent, not only in his being born of the free woman, but also in the higher rank of those who would come after him. Whereas Isaac would beget kings, Ishmael would beget princes.

For the first time, God set the time when the son would be born. We may wonder why God made Abraham wait such a long time for the fulfillment of the glorious promise; but it was absolutely imperative that the father of the Chosen People should truly believe God and know of a certainty that God's promises would be fulfilled, no matter how impossible and unreasonable they might have seemed from the human standpoint. "God fulfills his promises, not because they are reasonable by human standards, but because God is God, and His Word is true and absolutely reliable."[26]