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.
Source: Tractates on the Gospel of John (New Advent)