John Calvin Commentary Genesis 18:13

"And Jehovah said unto Abraham, Wherefore did Sarah laugh, saying, Shall I of a surety bear a child, who am old?" — Genesis 18:13 (ASV)

And the Lord said. Because the majesty of God had now been manifested in the angels, Moses expressly mentions His Name. We have previously declared in what sense the name of God is transferred to the angel; therefore, it is not necessary to repeat it now, except that it is always important to remark that the word of the Lord is so precious to Himself that He would be regarded by us as present whenever He speaks through His ministers.

Again, whenever He manifested Himself to the fathers, Christ was the Mediator between Him and them, who not only personates God in proclaiming His word but is also truly and essentially God. And because the laughter of Sarah had not been detected by the eye of man, Moses therefore expressly declares that she was reprehended by God.

The following circumstances also pertain to this point: that the angel had his back turned to the tent, and that Sarah laughed within herself and not before others. The censure also shows that Sarah's laughter was joined with incredulity. For there is considerable weight in this sentence: ‘Can anything be wonderful with God?’ But the angel chides Sarah because she limited the power of God within the bounds of her own understanding.

An antithesis is therefore implied between the immense power of God and the contracted measure Sarah conceived for herself through her carnal reason. Some translate the word פלא (pala) as hidden, as if the angel meant that nothing was hidden from God, but the sense is different: namely, that the power of God should not be estimated by human reason. It is not surprising that in arduous affairs we fail, or that we succumb to difficulties; but God’s way is very different, for He looks down with contempt from above on those things that alarm us by their lofty elevation.

We now see what Sarah's sin was: namely, that she wronged God by not acknowledging the greatness of His power. And truly, we also attempt to rob God of His power whenever we distrust His word. At first sight, Paul seems to give cold praise to Abraham's faith in saying that he did not consider his body, now dead, but gave glory to God because he was persuaded that He could fulfill what He had promised (Romans 4:19).

But if we thoroughly investigate the source of distrust, we will find that the reason we doubt God’s promises is because we sinfully detract from His power. For as soon as any extraordinary difficulty occurs, then whatever God has promised seems to us unbelievable; indeed, the moment He speaks, the perverse thought insinuates itself: How will He fulfill what He promises?

Being constrained and preoccupied by such narrow thoughts, we exclude His power, the knowledge of which is better to us than a thousand worlds. In short, whoever does not expect more from God than he is able to comprehend within the limited measure of his own reason does God grievous wrong.

Meanwhile, the word of the Lord should be inseparably joined with His power, for nothing is more preposterous than to inquire what God can do, to the setting aside of His declared will. In this way the Papists plunge themselves into a profound labyrinth when they dispute concerning the absolute power of God. Therefore, unless we are willing to be involved in absurd speculations, it is necessary that the word should precede us like a lamp, so that His power and His will may be conjoined by an inseparable bond. The Apostle prescribes this rule to us when he says:

‘Being certainly persuaded, that what He has promised,
He is able to perform,’ (Romans 4:21).

The angel again repeats the promise that he would come ‘according to the time of life,’ that is, in the revolving of the year, when the full time for giving birth would have arrived.