When he speaketh falsehood. These words are generally explained as if Christ affirmed that the blame of falsehood does not belong to God, who is the Author of nature, but, on the contrary, proceeds from corruption. But I explain it more simply, that it is customary with the devil to speak falsehood , and that he knows nothing but to contrive corruptions, frauds, and delusions. And yet we justly infer from these words, that the devil has this vice from himself, and that, while it is peculiar to him, it may likewise be said to be accidental; for, while Christ makes the devil to be the contriver of lying , he evidently separates him from God, and even declares him to be contrary to God. For he is a liar , and the father of it The word father has the same object as the preceding statement; for the reason why Satan is said to be the father of falsehood is, because he is estranged from God, in whom alone truth dwells, and from whom it flows as from the only fountain.
John Calvin, Commentary on the Gospel of John, Vol. 1 8.8