Then Abraham fell upon his face
In reverence of the divine Being, and as amazed at what was told him:
not through distrust and diffidence of the promise, as Sarah did, for he staggered not at that through unbelief, but for joy at such good news; and so Onkelos renders it, "and he rejoiced", with the joy of faith; it may be our Lord refers to this in ( John 8:56 ) ; he saw Christ in the promise of Isaac, and rejoiced that he should spring from his seed: the Targums of Jonathan and Jerusalem paraphrase it, "and he wondered"; he was amazed at the grace of God that gave him such a promise, and he was astonished at the power of God that must be exerted in the fulfilment of it: and therefore it follows,
and said in his heart;
within himself, without expressing anything as to be heard and understood by any creature; but the omniscient God knew what he said, and the language of it, whether of unbelief or not:
shall [a child] be born unto him that is an hundred years old?
not that he was now a full an hundred years old, he was ninety nine, and going in his hundredth year; but then he would be, as he was, an hundred years old when this child was born to him, ( Genesis 21:5 ) . It had been no unusual thing for a child to be born to a man when an hundred, and even many hundred years old, but it was so in Abraham's time; though indeed after this we read that Abraham himself had six sons by Keturah, when, his natural strength was afresh invigorated, and his youth was renewed like the eagle's; and besides Abraham said this, not so much with respect to himself, though his age was a circumstance that served to heighten the wonder, as with respect to Sarah, and the circumstances in which she was, who was to bear this son to him:
and shall Sarah, that is ninety years old, bear?
and with whom it had ceased to be after the manner of women, which made it more difficult of belief how it could be. Some think that Abraham said this, as somewhat doubting of it, until he was more strongly assured by the Lord that so it would be indeed, as is expressed in ( Genesis 17:19 ) ; but meeting with no reproof for what he said and did, as Sarah, it seems to show the contrary.