“It is impossible for God to lie” (Heb. 6[:18]). Therefore Moses states that God gave Abraham an heir by the lady of his house, Sarah, “as He had said.” Moses takes care to repeat this thrice [in Genesis 21]. Oh dear God, help my foolish heart that I may never doubt Your trustworthy pledge. “All Your promises in Christ are Certainty, Yea, and Amen” [2 Cor. 1:20].
Yet notice, dear heart, that Abraham was made to wait twenty-five years for Isaac, counting from the first pledge. God is not always the “Lord of Rushmont,” but often the “Lord of Whilemont and Waitmont.” Therefore, dear heart, if God does not leap up at once, just wait. “Wait for the LORD; be of good cheer and do not despair, and wait for the LORD” [Ps. 27:14}.
The fittest time to give He knows,
Nor ever harmful cunning shows.
Herberger, Valerius The Great Works of God: Parts Three and Four: The Mysteries of Christ in the Book of Genesis, Chapters 16-50 61