Or thus: The devil is not a singular, but a common name. In whomsoever the works of the devil are found, he is to be called the devil. It is the name of a work, not of a nature. Here then our Lord means by the father of the Jews, Cain; whom they wished to imitate, by killing the Saviour: for he it was who set the first example of murdering a brother. That he spoke a lie of his own, means that no one sins but by his own will. And inasmuch as Cain imitated the devil, and followed his works, the devil is said to be his father.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: de Quæst. Nov. et Vet. Test. 2, 90