Through faith also Sarah herself received strength to conceive seed - The word “herself” here - αὐτὴ autē - implies that there was something remarkable in the fact that “she” should manifest this faith. Perhaps there may be reference here to the incredulity with which she at first received the announcement that she should have a child; Genesis 18:11, Genesis 18:13. Even “her” strong incredulity was overcome, and though everything seemed to render what was announced impossible, and though she was so much disposed to laugh at the very suggestion at first, yet her unbelief was overcome, and she ultimately credited the divine promise. The apostle does not state the authority for his assertion that the strength of Sarah was derived from her faith, nor when particularly it was exercised. The argument seems to be, that here was a case where all human probabilities were against what was predicted, and where, therefore, there must have been simple trust in God. Nothing else but “faith” could have led her to believe that in her old age she would have borne a son.
When she was past age - She was at this time more than ninety years of age; Genesis 17:17; compare Genesis 18:11.
Because she judged him faithful who had promised - She had no other ground of confidence or expectation. All human probability was against the supposition that at her time of life she would be a mother.