15 “He that is of God hears God's words: ye therefore hear them not, because you are not of God.” Here, again, it is not of their nature as men, but of their depravity, that you are to think. In this way they are of God, and yet not of God. By nature they are of God, in depravity they are not of God. Give heed, I pray you. In the gospel you have the remedy against the poisonous and impious errors of the heretics. For of these words also the Manicheans are accustomed to say, See, here there are two natures, — the one good and the other bad; the Lord says it.
What says the Lord? “You therefore hear me not, because you are not of God.” This is what the Lord says. What then, he rejoins, do you say to that? Hear what I say. They are both of God, and not of God. By nature they are of God: by depravity they are not of God; for the good nature which is of God sinned voluntarily by believing the persuasive words of the devil, and was corrupted; and so it is seeking a physician, because no longer in health. That is what I say. But you think it impossible that they should be of God, and yet not of God.
Hear why it is not impossible. They are of God, and yet not of God, in the same way as they are the children of Abraham, and yet not the children of Abraham. Here you have it. It is not as you say. Hearken to the Lord Himself; it is He that said to them, “I know that you are the children of Abraham.” Could there be any lie with the Lord? Surely not. Then is it true what the Lord said? It is true. Then it is true that they were the children of Abraham? It is true. But listen to Himself denying it.
He who said, “You are the children of Abraham,” Himself denied that they were the children of Abraham. “If you are Abraham's children, do the deeds of Abraham. But now ye seek to kill me, a man that tells you the truth, which I have heard from God: this did not Abraham. You do the works of your father,” that is, of the devil. How, then, were they both Abraham's children, and yet not his children? Both states He showed in them. They were both Abraham's children in their carnal origin, and not his children in the sin of following the persuasion of the devil.
So, also, apply it to our Lord and God, that they were both of Him, and not of Him. How were they of Him? Because He it was that created the man of whom they were born. How were they of Him? Because He is the Architect of nature,— Himself the Creator of flesh and spirit. How, then, were they not of Him? Because they had made themselves depraved. They were no longer of Him, because, imitating the devil, they had become the children of the devil.
Source: Tractates on the Gospel of John (New Advent)