John 10:6

Simple is the language of the saints, and far removed from the elaborateness of the Greeks: for God chose the foolish things of the world, according to the word of Paul, that He might put to shame them that are wise. He used therefore the name of proverb, for thus he designates the parable, perhaps because the distinction of the two words was always somewhat confused, and the signification is understood equally well whether both or either be used. Yet this we do say, that the inspired Evangelist marvels much at the Jews' want of understanding. For as the experience of events itself bears witness, they have a mind like to rocks or to iron, persistently refusing to accept any profitable instruction of any sort. Wherefore it was said to them by the voice of Joel the Prophet: Rend your hearts and not your garments. And again, the writer of the Book seems to me not inconsiderately to have said: This parable spake Jesus unto them: but they understood not, he says, what things they were which He spake unto them; and he utters this with no little emphasis. For it is just the same as if he said plainly: So far are the Pharisees from being able to understand any necessary matter, although absurdly wise in their own conceits, that they understood not this parable, so clear to see, and so transparent, in which there is nothing hard to lay hold of, or tortuous to follow, or difficult to comprehend. And with propriety he mocks at the ill counsel of the Jews, since Christ appeared of no account to them, although He taught what was higher than the Law, and exhibited a system of instruction much more pleasing than that of Moses.

(Tr. xlv. 10. ct seq.) But here is a difficulty. Sometimes they who are not sheep hear Christ's voice; for Judas heard, who was a wolf. And sometimes the sheep hear Him not; for they who crucified Christ heard not; yet some of them were His sheep. You will say, While they did not hear, they were not sheep; the voice, when they heard it, changed them from wolves to sheep. Still I am disturbed by the Lord's rebuke to the shepherds in Ezekiel, Neither have ye brought again that which strayed. (Ezek. 34:4) He calls it a stray sheep, but yet a sheep all the while; though, if it strayed, it could not have heard the voice of the Shepherd, but the voice of a stranger. What I say then is this; The Lord knoweth them that are His. (2 Tim. 2:19) He knoweth the foreknown, he knoweth the predestinated. They are the sheep: for a time they know not themselves, but the Shepherd knows them; for many sheep are without the fold, many wolves within. He speaks then of the predestinated. And now the difficulty is solved. The sheep do hear the Shepherd's voice, and they only. When is that? It is when that voice saith, He that endureth to the end shall be saved. (Mat. 10:32) This speech His own hear, the alien hear not. (ut sup.) Our Lord feedeth by plain words, exerciseth by obscure. For when two persons, one godly, the other ungodly, hear the words of the Gospel, and they happen to be such that neither can understand them; one says, What He saith is true and good, but we do not understand it: the other says, It is not worth attending to. The former, in faith, knocks, yea, and, if he continue to knock, it shall be opened unto him. The latter shall hear the words in Isaiah, If ye will not believe, surely ye shall not be established. (Isa. 7:9)