Not meaning thereby that they had not descended in their natural course from Abraham, but that it avails them nothing to have Abraham for their father, unless they observed the relationship in respect of virtue. For Scripture is accustomed to entitle laws of relationship, such as do not exist by nature, but are derived from virtue or vice. To whichsoever of these two a man conforms himself, he is called its son or brother.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: ubi sup.