Our Lord says this with a view to put down their vain boasting of their descent; and persuade them to vest their hopes of salvation no longer on the natural relationship, but on the adoption. For this it was which prevented them from coming to Christ; viz. their thinking that their relationship to Abraham was sufficient for their salvation.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Hom. liv. 2