John Calvin Commentary Hebrews 11:12

"wherefore also there sprang of one, and him as good as dead, [so many] as the stars of heaven in multitude, and as the sand, which is by the sea-shore, innumerable." — Hebrews 11:12 (ASV)

Therefore sprang there even of one, etc. He now also reminds the Jews that it was by faith that they were the descendants of Abraham; for he was, as it were, half dead, and Sarah his wife, who had been barren in the flower of her age, was now sterile, being far advanced in years. Sooner, then, might oil be expected to flow from a stone than a nation to proceed from them: and yet there sprang from them an innumerable multitude.

If the Jews are now proud of their origin, let them consider what it was. Whatever they are, everything is doubtless to be ascribed to the faith of Abraham and Sarah. Hence, it follows that they cannot retain and defend the position they have acquired in any other way than by faith.