1 Our Lord, in the form of a servant, yet not a servant, but even in servant-form the Lord (for that form of flesh was indeed servant-like; but though He was “in the likeness of sinful flesh,” yet was He not sinful flesh) promised freedom to those who believed in Him. But the Jews, as if proudly glorying in their own freedom, refused with indignation to be made free, when they were the servants of sin. And therefore they said that they were free, because Abraham's seed.
What answer, then, the Lord gave them to this, we have heard in the reading of this day's lesson. “I know,” He said, “that you are Abraham's children; but you seek to kill me, because my word takes no hold in you.” I recognize you, He says; “You are the children of Abraham, but you seek to kill me.” I recognize the fleshly origin, not the believing heart. “You are the children of Abraham,” but after the flesh. Therefore He says, “You seek to kill me, because my word takes no hold in you.”
If my word were taken, it would take hold: if you were taken, you would be enclosed like fishes within the meshes of faith. What then means that— “takes no hold in you”? It takes not hold of your heart, because not received by your heart. For so is the word of God, and so it ought to be to believers, as a hook to the fish: it takes when it is taken. No injury is done to those who are taken; since they are taken for salvation, and not for destruction. Hence the Lord says to His disciples: “Come after me, and I shall make you fishers of men.” But such were not these; and yet they were the children of Abraham—children of a man of God, unrighteous themselves. For they inherited the fleshly genus, but had become degenerate, by not imitating the faith of him whose children they were.
Source: Tractates on the Gospel of John (New Advent)