“Sarai was barren; she had no child.” (Gen 11:30) She had no hope for Messiah to come from her womb. The first of three revelations was made at Mamre where they pitched their tents and Abram built an altar to the LORD (Gen 13:18). And then the Word of LORD appeared to him in a vision. He had no children and thus supposed that the promised seed was a spiritual seed only, even Eliezer of Damascus, a man who had attached himself by faith to the patriarch. But the Word of Yahweh confirmed to Abram that “a son who is your own flesh and blood will be your heir.” And the LORD “took him outside and said, ‘Look up at the sky and count the stars—if indeed you can count them.’ Then He said to him, ‘So shall your offspring be.’ Abram believed Yahweh, and he credited it to him as righteousness.” (Gen 15:4-6)
Yet Sarai was still unable to conceive and they applied human reasoning. Perhaps Sarai was not meant to be the mother. Perhaps God’s Word would be accomplished though Hagar, a maid servant. Ishmael was born from that union. He was the fruit of human striving to accomplish the promise of God. When Abram was ninety-nine years old, the LORD appeared to Abram and said to him, “I am Almighty God; walk before Me and be blameless. And I will make My covenant between Me and you, and will multiply you exceedingly.” Then Abram fell on his face…” Did you hear the call to be perfect and Abram’s helpless response. And the LORD took occasion to clarify the issue of seed and give them new names— Abraham—“for I have made you a father of many nations. “ (Gen 17:1-5)— and — “As for Sarai your wife, you are no longer to call her Sarai; her name will be Sarah. I will bless her and will surely give you a son by her. I will bless her so that she will be the mother of nations; kings of peoples will come from her." (Gen 17:15-16) So here Sarai was included in the fufiulment of the promise of a son.
And most miraculously, I have come to believe that, next, the Lord Jesus manifested Himself to Abram by the terebinth trees of Mamre, as he was sitting in the tent door in the heat of the day. The Scriptures says, “So he lifted his eyes and looked, and behold, three men were standing by him; and when he saw them, he ran from the tent door to meet them, and bowed himself to the ground.” (Gen 18:1-2) "He visits... for the twofold purpose of drawing out and completing the faith of Sarah, and of communing with Abraham concerning the destruction of Sodom.” (Barnes) This is "another manifestation of the divine presence, more familiar than any yet narrated; and more like that in the fulness of time, when the Word was made flesh.” (Jamieson-Fausset-Brown) "The three men were three messengers,… one of whom was the Lord Christ, Malachi 3:1...” (Joseph Sutcliffe)
Jesus and His angels asked, “Where is Sarah your wife?” So he said, “Here, in the tent.” "And He told her, 'I will certainly return to you according to the time of life, and behold, Sarah your wife shall have a son.' (Sarah was listening in the tent door which was behind him.) Now Abraham and Sarah were old, well advanced in age; and Sarah had passed the age of childbearing. Therefore Sarah laughed within herself, saying, 'After I have grown old, shall I have pleasure, my lord being old also?' Then Yahweh said to Abraham: ‘Why did Sarah laugh and say, ‘Will I really have a child, now that I am old?’ Is anything too hard for the Lord? I will return to you at the appointed time next year, and Sarah will have a son.’ Sarah was afraid, so she lied and said, ‘I did not laugh.’ But he said, ‘Yes, you did laugh.’” (Gen 18:9-15) She laughed inwardly and this knowledge caused her to trust.
“After being thus made aware of who promised them a son, Sarah, believed it; hence the truth of these words, ‘By faith even Sarah, herself also received strength to conceive seed, and she bore a child when she was past the age, because she judged Him faithful who had promised.’” (Heb 11:11) .’” (Burton Coffman) “I believe, that this was not a mere miraculous generation, but that she received a general restoration of her nature for the production of a child, which was before decayed...” (John Owen)
Sarah “judged Him faithful who had promised.”— This is her act in the Hebrews hall of fame. “The act which is here ascribed unto Sarah is, that she ‘judged’ or reckoned, reputed and esteemed, God to be faithful: she was assured that He would make good His Word, on which He had caused her to hope.” (A. W. Pink) So it came to pass that at the appointed time, “Sarah conceived and bore a son to Abraham in his old age, at the appointed time of which God had spoken to him.” (Gen 21:2)
“She laughed; but she also persevered. And at Isaac’s birth, Sarah declared that she now had a proper reason to laugh (Gen 21:6). She had great joy because God had carried out his promise to her (Gen 21:1). Yahweh had done what they were unable to contrive.
[The lesson:] God is not tied down to the order of nature, nor limited by any secondary causes. He will turn nature upside down rather than not be as good as His Word. He has brought water out of a rock, made iron to float (2 Kings 6:6), sustained two million people in a howling wilderness… ‘Therefore from one man, and him as good as dead, were born as many as the stars of the sky in multitude—innumerable as the sand which is by the seashore.’ (Heb 11:12) We regard this 12th verse as setting forth the fruit of her faith, namely the numerous posterity which issued from her son, Isaac. The double reference to the ‘sand’ and the ‘stars’ calls attention to the twofold seed: the earthly and the heavenly, the natural and the spiritual Israel.” (A. W. Pink) This language is the same used by Paul to describe the justification of Abraham.