2 You have heard, indeed, the Lord saying, “I know that you are Abraham's children.” Hear what He says afterwards: “I speak that which I have seen with my Father; and you do that which you have seen with your father.” He had already said, “I know that you are Abraham's children.” What is it, then, that they do? What He told them: “You seek to kill me.” This they never saw with Abraham. But the Lord wishes God the Father to be understood when He says, “I speak that which I have seen with my Father.” I have seen the truth: I speak the truth, because I am the Truth. For if the Lord speaks the truth which He has seen with the Father, He has seen Himself— He speaks Himself; because He Himself is the Truth of the Father, which He saw with the Father. For He is the Word— the Word which was with God. The evil, then, which these men do, and which the Lord chides and reprehends, where have they seen it? With their father. When we come to hear in what follows the still clearer statement who is their father, then shall we understand what kind of things they saw with such a father; for as yet He names not their father. A little above He referred to Abraham, but in regard to their fleshly origin, not their similarity of life. He is about to speak of that other father of theirs, who neither begot them nor created them to be men. But still they were his children in as far as they were evil, not in as far as they were men; in what they imitated him, and not as created by him.
3. “They answered and said unto Him, Abraham is our father;” as if, What have you to say against Abraham? Or, If you can, dare to find fault with Abraham. Not that the Lord dared not find fault with Abraham; but Abraham was not one to be found fault with by the Lord, but rather approved. But these men seemed to challenge Him to say some evil of Abraham, and so to have some occasion for doing what they purposed. “Abraham is our father.”
Source: Tractates on the Gospel of John (New Advent)