1 And the Lord visited Sarah as He had said, and the Lord did for Sarah as He had spoken.
2 For Sarah conceived and bore Abraham a son in his old age, at the set time of which God had spoken to him.
3 And Abraham called the name of his son who was born to him—whom Sarah bore to him—Isaac.
4 Then Abraham circumcised his son Isaac when he was eight days old, as God had commanded him.
5 Now Abraham was one hundred years old when his son Isaac was born to him.
6 And Sarah said, “God has made me laugh, and all who hear will laugh with me.”
7 She also said, “Who would have said to Abraham that Sarah would nurse children? For I have borne him a son in his old age.”
8 So the child grew and was weaned. And Abraham made a great feast on the same day that Isaac was weaned.
In spite of all Satan’s efforts Isaac, the progenitor and type of the Messiah is born; and born at the “set time,” that is, the “exact time” of which God had spoken to Abraham. … His birth was a miracle; but at the same time it must not be forgotten that human life at that period was very much longer than today, so that a woman of 90 was not at all so aged as would appear in present times. Sarah’s father lived for 200 years; at the time of Isaac’s birth Shem was about 560 years of age; and there must have been many thousands of persons at the time who had already lived through two or three centuries.
The chapter presents a new creation, hence the Divine title “Elohim” and not “Jehovah” appears throughout it, until verse 33 is reached. In this verse God is “Jehovah” because it touches His covenant relationship as a Savior….
Isaac means “laughter.” His advent filled the mouth of Sarah with laughter, and caused all that heard to laugh with her. — Williams, page 25.
Finally, after twenty-five years in the land of Canaan, waiting for the fulfillment of God’s promise, Abraham’s faith was rewarded. The apostle Paul has a beautiful commentary on this.
(As it is written, “I have made you a father of many nations”) in the presence of Him whom he believed—God, who gives life to the dead and calls those things which do not exist as though they did; who, contrary to hope, in hope believed, so that he became the father of many nations, according to what was spoken, “So shall your descendants be.” And not being weak in faith, he did not consider his own body, already dead (since he was about a hundred years old), and the deadness of Sarah’s womb. 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. And therefore “it was accounted to him for righteousness” (Romans 4:17-22).
Abraham, therefore, is offered throughout the New Testament as the prototype of those who are justified by faith. The apostle Paul stresses that this imputed righteousness was received by Abraham even before he was circumcised (Romans 4:9-12) and long before the Mosaic law was given (Galatians 3:16-18), so that salvation depends neither on the ritual nor the moral law, but only on God’s grace received through faith. Abraham is called “the father of all them that believe” (Romans 4:11) and “the father of us all” (Romans 4:16). Further, Scripture assures us that “they which are of faith, the same are the children of Abraham” and that “they which be of faith are blessed with faithful Abraham” (Galatians 3:7, 9).
Sarah also, though of doubtful mind for a time, became strong in faith. “Through faith also Sara herself received strength to conceive seed, and was delivered a a child when she was past age, because she judged him faithful who had promised” (Hebrews 11:11). — Morris, pages 365-366.
Sarah was “young” enough again, not only to have a child, but to nurse him (verse 7); Abraham was “young” enough again not only to father Isaac, but also six other sons of his wife Keturah, after Sarah died (Genesis 25:2)! When God miraculously heals, it is not a partial healing, but a complete and instant restoration. — Morris, page 366-367.
There is such a thing as God’s “set time”—His “due season,” and for this the faithful must be content to wait. The time may seem long, and hope deferred may make the heart sick; but the spiritual mind will ever find its relief in the assurance that all is for the ultimate display of God’s glory. “For the vision is for an appointed time, but at the end it shall speak and not lie; though it tarry, wait for it; because it will assuredly come, it will not tarry … but the just shall live by his faith” (Habakkuk 2:3-4). — Mackintosh, page 217.