Gen 17:17

“Now Abraham was not guilty of any doubt by his laughter, for he showed his love toward Ishmael in what he said. He had clung to this hope for twenty-five years. Abraham had manifested his faith in every vision that had come to him. However great his contest with barrenness became, he manifested the victory of his faith. But when old age was added to the barrenness, he laughed in his heart. That his Lord would do these two things for him was a marvel to him.”

“One should consider the fact that Abraham was uncircumcised when God called him, and he was still uncircumcised when he was promised a legitimate son as heir. You are hereby invited to believe that he is not only the father of the Jews, as they claim, but the progenitor, through faith, of all believers. Sarah also, before the circumcision of her husband and by the addition of one letter to her name, receives the blessing of no small gift, so that she might have the primacy of virtue and of grace. God promises that from her nations and kings of peoples will come, so that in her might be established the type not of the synagogue but of the church. The fact that Abraham laughed when he had been promised a son through her was an expression not of unbelief but of joy. Indeed, he "fell on his face"—in worship, which means he believed. And he added, "Shall a child be born to a man who is a hundred years old? Shall Sarah, who is ninety years old, bear a child?" And he said, "O that Ishmael might live in thy sight!" He is not incredulous with regard to the promises, nor is he greedy in what he asks for in prayer. "I have no doubt that you will come through, granting a son to an old man of a hundred years and that, as the author of nature, you will effectively stretch its limits. Blessed indeed is the one on whom this gift is bestowed; but I will be doubly favored if even this Ishmael here, whom I begot from the household slave, should live in your presence." And so the Lord approved Abraham's sentiments, did not deny his request and confirmed his own promises.”

“(Verse 17.) And Abraham fell upon his face and laughed. And he said in his heart, Shall a child be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear? And after a little while: And thou shalt call his name Isaac. Different opinions, but there is one etymology why he is called Isaac. For Isaac is interpreted as laughter. Others say that Sarah laughed, therefore he was called laughter, which is false. Others indeed say that Abraham laughed, which we also approve. After Ishmael was born, Isaac was born, and then we read about Abraham and Sarah laughing. However, it should be known that in the Old Testament, four people were called by their names before they were born, without any disguise: Ishmael, Isaac, Solomon, and Josiah. Read the Scriptures.”

“Abraham fell on his face, and laughed, saying in his heart: "Shall a child be born to a man who is a hundred years old, and will Sarah, who is ninety years old, bear a child?" The laughter of Abraham is the exultation of one rejoicing, not the mockery of one disbelieving: those words which he said in his heart are not those of one doubting, but of one admiring; as testified also by the Apostle who, speaking of him, said: "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." (Romans 4:20).”

“ויפל אברהם על פניו ויצחק AND ABRAHAM FELL UPON HIS FACE AND LAUGHED — This word ויצחק Onkelos translates by וחדי which signifies joy (“and he rejoiced’’) but the similar verb in the case of Sarah (ותצחק Genesis 18:12) he translates as meaning laughter. From this you may understand that Abraham had faith and rejoiced, and that Sarah had no faith and sneered, and that is why God was angry with Sarah (when she laughed) but was not angry with Abraham. הלבן SHALL A CHILD BE BORN TO ONE WHO IS A HUNDRED YEARS OLD? — There are some questions expressing astonishment which do not imply a doubt but rather affirm the certainty that the circumstance referred to has happened or will happen, as (1 Samuel 2:2) “Did I reveal myself [unto thy father’s house?]” (Of course, I did) and (Ezekiel 8:6) “Seest thou what they do?” (You, of course, have seen it). This question, too, affirms the certainty that it is possible for God to cause Abraham, at the age of hundred, to beget a son and this is in effect what he said to himself: Would there be done to another man one hundred years old this favour that the Holy One, blessed be He, is doing to me”? (Rashi means that the words must be translated, “would a child be born to any other person who is a hundred years old?”). ואם שרה הבת תשעים שנה OR SHALL SARAH, WHO IS NINETY YEARS OLD be worthy to bear a son? Although the previous generations used to beget at the age of five hundred years, yet in Abraham’s days the length of life had already become shorter and a diminution of strength had come upon people. Go and learn this from the ten generations from Noah to Abraham who begot their children at the then early age of sixty or seventy.”

“‘VAYITZCHAK’ (AND HE LAUGHED). Onkelos translated: “and he rejoiced.” This is correct since the word tzachak can be used intermittently for sporting or rejoicing, just as in the verses: ‘Mesacheketh’ (Sporting) in His habitable earth, ‘mesachkim’ (rejoiced) before the Eternal. In my opinion the intent of this expression is to convey the thought that whoever sees a favorable unusual event in one’s life rejoices to the point where “his mouth is filled with laughter.” It is this thought which Sarah expressed: G-d hath made laughter for me; everyone that heareth will laugh on account of me, just as in the verse, Then was our mouth filled with laughter, and our tongue with singing. And this is what Abraham did. When this good tiding was related to him, he rejoiced and his mouth was filled with laughter, and he said in his heart that this is an occasion for rejoicing as it is a very wonderful matter. Shall a child be born unto him that is a hundred years old? and shall Sarah, that is ninety years old, bear? And will this matter not give cause for rejoicing and happiness? Only, Scripture speaks summarily of his wonderment, as it is connected with the word vayitzchak. A similar case is the expression, Have I even here seen, which is connected with the beginning of the verse: for she [Hagar] said, “Have I even here seen G-d, revealing to me that He saw in my affliction, and shall I not call Him E-il-ro’i [the G-d who seeth me]?” Another such example is the expression, Did I reveal Myself unto the house of thy father, which is connected with the following verse, Why do you kick at My sacrifice and at Mine offering,, Verse 29. thus stating, “Have I chosen you so that you kick at My sacrifice and Mine offering? Why then do you do so?” It may be that the question, Shall a child be born to him that is a hundred years old? is an expression of amazement, not one of impossibility, just as in the verse, Wilt thou judge the bloody city, which means, “Do you want to judge her and cause her to know all her abominations?” A similar case is the verse, Wilt thou set thine eyes upon it? it is gone. Likewise is the verse, Hast thou eaten of the tree, whereof I commanded thee that thou shouldst not eat? That is to say, “Has the thought occurred to you to eat of the tree?” This verse likewise is saying, “Has the thought occurred to anyone that a child be born to him that is a hundred years old, and that Sarah, who is ninety years, shall bear?” After that Abraham said to Him that he wishes this miracle be with the life of Ishmael. SHALL A CHILD BE BORN TO HIM THAT IS A HUNDRED YEARS OLD? There is no wonder that a person who is a hundred years old should beget children, for men beget as long as they have vigor even if they be ninety years old or a hundred years old, even in these generations. All the more in the days of Abraham, who had not yet lived two thirds of his life. Moreover, forty years after this, he begot many children from Keturah. But Abraham’s saying, Shall a child be born to him that is a hundred years old, expressed this thought: Since he did not beget a child from this woman while he was young, how could he beget from her after he was a hundred and she ninety? He knew that her womanly periods had ceased. Therefore Abraham did not say so originally when he was told, And I will make nations of thee, but only when he was told that he would beget a child from Sarah. Now He commanded him, And thou shalt call his name Isaac, on account of the rejoicing done by Abraham, which is proof that it was out of faith and joy [that Abraham said, Shall a child be born, etc.] And after Abraham called him so, as G-d had commanded him, Sarah said, “Is he not rightly named Isaac? For G-d hath made laughter for me.””