13 Hear and understand. I shall not send you far away [for the meaning]; understand it from the words themselves. The Lord called the devil the father of falsehood. What is this? Hear what it is, only revolve the words themselves, and understand. It is not every one who tells a lie that is the father of his lie. For if you have got a lie from another, and uttered it, thou indeed hast lied in giving utterance to the lie; but you are not the father of that lie, because you have got it from another.
But the devil was a liar of himself. He begot his own falsehood; he heard it from no one. As God the Father begot as His Son the Truth, so the devil, having fallen, begot falsehood as his son. Hearing this, recall now and reflect upon the words of the Lord. You catholic minds, consider what you have heard; attend to what He says. “He”— who? The devil— “was a murderer from the beginning.” We admit it—he slew Adam. “And he abode not in the truth.” We admit it, for he lapsed from the truth.
“Because there is no truth in him.” True: by falling away from the truth he has lost its possession. “When he speaks a lie, he speaks of his own: for he is a liar, and the father of it.” He is both a liar, and the father of lies. For thou, it may be, are a liar, because you utter a lie; but you are not its father. For if you have got what you say from the devil, and hast believed the devil, you are a liar, but not the father of the lie. But he, because he got not elsewhere the lie wherewith in serpent-form he slew man as if by poison, is the father of lies just as God is Father of truth. Withdraw, then, from the father of lies: make haste to the Father of truth; embrace the truth, that you may enter into liberty.
Source: Tractates on the Gospel of John (New Advent)