11 That question has been solved in a way, and perhaps satisfies every one. But I bare still a subject of concern, and what concerns me I shall impart to you, that, in some sort inquiring together, I may through His revelation be found worthy with you to attain the solution. Hear, then, what it is that moves me. By the Prophet Ezekiel the Lord rebukes the shepherds, and among other things says of the sheep, “The wandering sheep have ye not recalled.” He both declares it a wanderer, and calls it a sheep.
If, while wandering, it was a sheep, whose voice was it hearing to lead it astray? For doubtless it would not be straying were it hearing the shepherd's voice: but it strayed just because it heard another's voice; it heard the voice of the thief and the robber. Surely the sheep do not hear the voice of robbers. “Those that came,” He said—and we are to understand, apart from me—that is, “those that came apart from me are thieves and robbers, and the sheep did not hear them.”
Lord, if the sheep did not hear them, how can the sheep wander? If the sheep hear only You, and You are the truth, whoever hears the truth cannot certainly fall into error. But they err, and are called sheep. For if, in the very midst of their wandering, they were not called sheep, it would not be said by Ezekiel, “The wandering sheep have ye not recalled.” How is it at the same time a wanderer and a sheep? Has it heard the voice of another? Surely “the sheep did not hear them.”
Accordingly many are just now being gathered into Christ's fold, and from being heretics are becoming Catholics. They are rescued from the thieves, and restored to the shepherds: and sometimes they murmur, and become wearied of Him that calls them back, and have no true knowledge of him that would murder them; nevertheless also, when, after a struggle, those have come who are sheep, they recognize the Shepherd's voice, and are glad they have come, and are ashamed of their wandering.
When, then, they were glorying in that state of error as in the truth, and were certainly not hearing the Shepherd's voice, but were following another, were they sheep, or were they not? If they were sheep, how can it be the case that the sheep do not listen to aliens? If they were not sheep, wherefore the rebuke addressed to those to whom it is said, “The wandering sheep have ye not recalled”? In the case also of those already become catholic Christians, and believers of good promise, evils sometimes occur: they are seduced into error, and after their error are restored.
When they were thus seduced, and were rebaptized, or after the companionship of the Lord's fold were turned back again into their former error, were they sheep, or were they not? Certainly they were Catholics. If they were faithful Catholics, they were sheep. If they were sheep, how was it that they could listen to the voice of a stranger when the Lord says, “The sheep did not hear them”?
Source: Tractates on the Gospel of John (New Advent)