"You are of your father the Devil."
Context! Christ was very much saying that these people had no faith in God at all! John 8:37-47
37 I know that ye are Abraham's seed; but ye seek to kill me, because my word hath no place in you.
38 I speak that which I have seen with my Father: and ye do that which ye have seen with your father.
39 They answered and said unto him, Abraham is our father. Jesus saith unto them, If ye were Abraham's children, ye would do the works of Abraham.
40 But now ye seek to kill me, a man that hath told you the truth, which I have heard of God: this did not Abraham.
41 Ye do the deeds of your father. Then said they to him, We be not born of fornication; we have one Father, even God.
42 Jesus said unto them, If God were your Father, ye would love me: for I proceeded forth and came from God; neither came I of myself, but he sent me.
43 Why do ye not understand my speech? even because ye cannot hear my word.
44 Ye are of your father the devil, and the lusts of your father ye will do. He was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father of it.
45 And because I tell you the truth, ye believe me not.
46 Which of you convinceth me of sin? And if I say the truth, why do ye not believe me?
47 He that is of God heareth God's words: ye therefore hear them not, because ye are not of God.
OSB Commentary:
8:37-40 To be a child of Abraham, it is not enough to simply be related by blood; rather, Abraham's true children share his faith and virtue (Lk 3:8). St. John Chrysostom teaches that our Lord wanted to detach the Jews from racial pride and to teach them no longer to place their hope of salvation in being of the race of Abraham's children by nature, but to come to faith by their own free will. Their idea that being a descendant of Abraham was enough for salvation was the very thing that prevented them from coming to Christ.
8:42 Proceeded refers not to the Son coming eternally from the Father, but to Christ being sent from the Father to His Incarnation on earth.
8:44 Just as being a child of Abraham is based on sharing his attributes (see note for vv. 37-40), like-wise, those who reject Christ share the same attributes as the devil (in particular, a hatred for truth), and thus are rightly called the devil's children.