6 Thus far, however, the Lord Jesus speaks in covert language; not as yet is He understood. He names the door, He names the sheepfold, He names the sheep: all this He sets forth, but does not yet explain. Let us read on then, for He is coming to those words, wherein He may think proper to give us some explanation of what He has said; from the explanation of which He will perhaps enable us to understand also what He has not explained. For He gives us what is plain, for food; what is obscure, for exercise. “He that enters not by the door into the sheepfold, but climbs up some other way.” Woe to the wretch, for he is sure to fall! Let him then be humble, let him enter by the door: let him walk on the level ground, and he shall not stumble. “The same,” He says, “is a thief and a robber.” The sheep of another he desires to call his own sheep—his own, that is, as carried off by stealth, for the purpose, not of saving, but of slaying them. Therefore is he a thief, because what is another's he calls his own; a robber, because what he has stolen he also kills. “But he that enters in by the door is the shepherd of the sheep: to him the porter opens.” Concerning this porter we shall make inquiry, when we have heard of the Lord Himself what is the door and who is the shepherd. “And the sheep hear his voice: and he calls his own sheep by name.” For He has their names written in the book of life. “He calls his own sheep by name.” Hence, says the apostle, “The Lord knows them that are His.” “And he leads them out. And when he puts forth his own sheep, he goes before them, and the sheep follow him: for they know his voice. And a stranger do they not follow, but do flee from him: for they know not the voice of strangers.” These are veiled words, full of topics of inquiry, pregnant with sacramental signs. Let us follow then, and listen to the Master as He makes some opening into these obscurities; and perhaps by the opening He makes, He will cause us to enter.
7. “This parable spoke Jesus unto them; but they understood not what He spoke unto them.” Nor we also, perhaps. What, then, is the difference between them and us, before even we can understand these words? This, that we on our part knock, that it may be opened unto us; while they, by disowning Christ, refused to enter for salvation, and preferred remaining outside to be destroyed. In as far, then, as we listen to these words with a pious mind, in as far as, before we understand them, we believe them to be true and divine, we stand at a great distance from these men. For when two persons are listening to the words of the gospel, the one impious, the other pious, and some of these are such as neither perhaps understands, the one says, It has said nothing; the other says, It has said the truth, and what it has said is good, but we do not understand it. This latter, because he believes, now knocks, that he may be worthy to have it opened up to him, if he continue knocking; but the other still hears the words, “If you believe not, you shall not understand.” Why do I draw your attention to this? Even for this reason, that when I have explained as I can these obscure words, or, because of their great abstruseness, I have either myself failed to arrive at an understanding of them, or wanted the faculty of explaining what I do understand, or every one has been so dull as not to follow me, even when I give the explanation, yet should he not despair of himself; but continue in faith, walk on in the way, and hear the apostle saying, “And if in anything ye be otherwise minded, God shall reveal even this unto you. Nevertheless whereto we have already attained, let us walk therein.”
Source: Tractates on the Gospel of John (New Advent)