10 Why is it, then, that I have said, This is a more important point? What is there about it obscure and difficult to understand? Listen, I beseech you. See, the Lord Jesus Christ Himself came and preached. Much more surely was that the Shepherd's voice which was uttered by the very mouth of the Shepherd. For if the Shepherd's voice came through the prophets, how much more did the Shepherd's own tongue give utterance to the Shepherd's voice? Yet all did not hear Him.
But what are we to think? Those who did hear, were they sheep? Lo? Judas heard, and was a wolf: he followed, but, clad in sheep-skin, he was laying snares for the Shepherd. Some, again, of those who crucified Christ did not hear, and yet were sheep; for such He saw in the crowd when He said, “When you have lifted up the Son of man, then shall you know that I am He.” Now, how is this question to be solved? They that are not sheep do hear, and they that are sheep do not hear. Some, who are wolves, follow the Shepherd's voice; and some, that are sheep, contradict it.
Last of all, the sheep slay the Shepherd. The point is solved; for some one in reply says, But when they did not hear, as yet they were not sheep, they were then wolves: the voice, when it was heard, changed them, and out of wolves transformed them into sheep; and so, when they became sheep, they heard, and found the Shepherd, and followed Him. They built their hopes on the Shepherd's promises, because they obeyed His precepts.
Source: Tractates on the Gospel of John (New Advent)