Genesis 17:21

With Isaac. Repeating the statement of v. 19, God assured Abraham that Isaac, not Ishmael, was to be the son of the covenant. While Ishmael should share generally in the blessings promised to Abraham, the offspring of Isaac would become sufficiently numerous to possess the land of Canaan. Specifically, the covenant, with all its material and spiritual blessings, was to Sarah’s son, Isaac, and to his posterity. The later history of the two sons fully justifies God’s selection of the one and rejection of the other. Even though Hagar had come to believe in the true God, the influence of her earlier Egyptian training proved to be decisive in the life of Ishmael and his sons, for his descendants became pagans.

At this set time. A time limit is now attached to the promise of a son. No longer could there be room for uncertainty. After waiting nearly 25 years since the first promise, and having shown faith and doubt in the past, Abraham learned that the time of waiting was soon to end.