Genesis 17:21

But My covenant will I establish with Isaac, which Sarah shall bear unto thee at this set time in the next year. God had, even now, appointed the time when Isaac should be born. And this child of promise should be the bearer of the Messianic promise. For since it stands to reason and clearly follows from the Scriptural account that Abraham taught all his children the fear of the true God, we must conclude that the special progeny of Abraham and Isaac to which the Lord here refers must be the sum total of that spiritual offspring, all men who by the faith of Abraham become the children of Abraham, Romans 4:11.