John Calvin Commentary Genesis 18:12

"And Sarah laughed within herself, saying, After I am waxed old shall I have pleasure, my lord being old also?" — Genesis 18:12 (ASV)

Therefore Sarah laughed within herself. Abraham had laughed before, as appears in the preceding chapter (Genesis 17:1); but the laughter of both was by no means similar. For Sarah is not overcome with admiration and joy on receiving the promise of God, but foolishly sets her own age and that of her husband in opposition to the word of God, so that she withholds confidence from God when He speaks.

Yet she does not openly charge God with falsehood or vanity; but because, having her mind fixed on contemplating what was proposed, she only weighs what might be accomplished by natural means, without raising her thoughts to consider God's power, and thus rashly casts discredit on God who speaks to her.

Thus, whenever we measure the promises and works of God by our own reason and by the laws of nature, we act disrespectfully toward Him, even if we intend nothing of the sort. For we do not give Him the honor He is due unless we regard every obstacle that presents itself in heaven and on earth as subject to His word.

But although Sarah's incredulity is not to be excused, she nevertheless does not directly reject the favor of God but is only so restrained by shame and modesty that she does not fully believe what she hears. Even her very words declare the greatest modesty: ‘After we are grown old shall we give ourselves up to lust?’ Therefore, let us observe that the furthest thing from Sarah's mind was making God a liar.

But her failing consisted in this alone: that, having fixed her thoughts too much on the accustomed order of nature, she did not give glory to God by expecting from Him a miracle that she was unable to conceive in her mind. We must here notice the admonition that the Apostle gathers from this passage, because Sarah here calls Abraham her lord (1 Peter 3:6).

For he exhorts women, following her example, to be obedient and well-behaved towards their own husbands. Many women, indeed, readily give their husbands this title, even while they do not hesitate to dominate them with their imperious pride; but the Apostle takes it for granted that Sarah testifies from her heart what she feels concerning her husband, nor is it doubtful that she proved through her actions the modesty she had professed in words.