6 Why, then, does this empty and vain bragging exalt itself? Let them cease boasting that they are the children of Abraham. They have heard what they ought to have heard: “If you are the children of Abraham,” prove it by your deeds, not by words. “You seek to kill me, a man;”— I say not, meanwhile, the Son of God; I say not God; I say not the Word, for the Word dies not. I say merely this that you see; for only what you see can you kill, and whom you see not can you offend. “This,” then, “did not Abraham.” “You do the works of your father.” And as yet He says not who is that father of theirs.
Source: Tractates on the Gospel of John (New Advent)