John Chrysostom

"Wherefore even from one they" all "sprang, as the stars of the sky, and as the sand which is by the sea-shore." "Wherefore" (he says) "even from one they" all "sprang." Here he not only says that she bare a child, but that she also became mother of so many as not even fruitful wombs are mothers of. "As the stars," He says. How then is it that He often numbers them, although He said, "As the stars of the heaven shall not be numbered, so neither shall your seed"? …

Homily on Hebrews 23 ↗

Theophylact of Ohrid

Faith accomplished not only that she gave birth, but that she gave birth to as many as even fertile women do not bear. Thus, two miracles. But in what sense were they innumerable? For they were often counted. Either this is said hyperbolically, according to the custom of Scripture, as for example: we saw cities reaching even to heaven (cf. Num. 13:29); or he said "innumerable" with reference to those who are constantly being born anew.

Commentary on Hebrews ↗

Thomas Aquinas

592. – Then when he says, for which cause there sprung even from one man, he shows what she obtained in her children by God's power, namely, a multiplying of seed in the merit of faith, from one, namely, Abraham, they were sprung: 'I called him alone, and blessed him, and multiplied him' (Is. 51:2). Secondly, his condition should be considered, because he was as good as dead; for he was already an old man, as was stated above. …

Commentary on Hebrews ↗