Genesis 17:17 - Clarke's commentary and critical notes on the Bible

Then Abraham fell upon his face, and laughed, and 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? Then Abraham - laughed - I am astonished to find learned and pious men considering this as a token of Abraham's weakness of faith or unbelief, when they have the most positive assurance from the Spirit of God himself that Abraham was not weak but strong in the faith; that he staggered not at the promise through unbelief, but gave glory to God, Romans 4:19, Romans 4:20. It is true the same word is used, Genesis 18:12, concerning Sarah, in whom it was certainly a sign of doubtfulness, though mixed with pleasure at the thought of the possibility of her becoming a mother; but we know how possible it is to express both faith and unbelief in the same way, and even pleasure and disdain have been expressed by a smile or laugh. By laughing Abraham undoubtedly expressed his joy at the prospect of the fulfillment of so glorious a promise; and from this very circumstance Isaac had his name. יצחק yitschak, which we change into Isaac, signifies laughter; and it is the same word which is used in the verse before us: Abraham fell on his face, ויצחק vaiyitschak, and he laughed; and to the joy which he felt on this occasion our Lord evidently alludes, John 8:56 : Your father Abraham Rejoiced to see my day; and he saw it, and was Glad. And to commemorate this joy he called his son's name Isaac. See note on Genesis 21:6.