John Calvin Commentary Genesis 17:17

"Then Abraham fell upon his face, and laughed, and said in his heart, Shall a child be born unto him that is a hundred years old? and shall Sarah, that is ninety years old, bear?" — Genesis 17:17 (ASV)

And Abraham fell upon his face. This was a sign, not only of his reverence, but also of his faith. For Abraham not only adores God, but in giving Him thanks, testifies that he receives and embraces what was promised concerning a son. From this we also infer that he laughed, not because he either despised, or regarded as fabulous, or rejected, the promise of God; but, as usually happens in things that are least expected, partly exulting with joy, and partly being carried beyond himself in admiration, he bursts into laughter.

For I do not agree with the opinion of those who suppose that this laughter flowed solely from joy; but I rather think that Abraham was astonished. His next question also confirms this: Shall a child be born to him that an hundred years old? For although he does not reject as vain what had been said by the angel, he yet shows that he was affected in no other way than as if he had received some incredible news.

The novelty of the thing so strikes him that for a short time he is bewildered. Yet he humbles himself before God, and with a confused mind, prostrating himself on the earth, he, by faith, adores the power of God. For Paul, in his Epistle to the Romans, is a witness that this was not the language of one who doubts (Romans 4:19). Paul denies that Abraham considered his body now dead, or the barren womb of Sarah, or that he staggered through unbelief, but declares that he believed in hope against hope.

Regarding what Moses relates—that Abraham said in his heart—I do not explain it as if he had distinctly conceived this in his mind. But, as many things come upon us contrary to our intentions, the perplexing thought suddenly rushed upon his mind: What a strange thing is this, that a son should be born to one a hundred years old! This, however, seems to some to be a kind of contest between carnal reason and faith. For although Abraham, reverently prostrating himself before God, submits his mind to the divine word, he is still disturbed by the novelty of the affair.

I answer that this admiration, which did not obstruct the course of God’s power, was not contrary to faith. Indeed, the strength of faith shone all the more brightly in having overcome such a difficult obstacle. And therefore he is not rebuked for laughing, as Sarah is in the next chapter (Genesis 18:1).