Christ reveals the marks whereby the sheep can distinguish a true shepherd from a deceiver and robber.
Identifying the marks of the deceiver and robber, Christ said, “Verily, verily, I say unto you, He that entereth not by the door into the sheepfold, but climbeth up some other way, the same is a thief and a robber” (Jn 10:1). St. John Chrysostom explained in his homily: “There is an allusion here to Antichrist, and to certain false christs who had been, and were to be. The Scriptures He calls the door. They admit us to the knowledge of God, they protect the sheep, they shut out the wolves, they bar the entrance to heretics. He that useth not the Scriptures, but climbeth up some other way, that is, some self-chosen, some unlawful way, is a thief.”
St. Augustine writes: “Many go under the name of good men according to the standard of the world, and observe in some sort the commandments of the Law. . . . And these generally boast of themselves. . . . Whoso wisheth to enter into the sheepfold, let him enter by the door; let him preach Christ; let him seek Christ’s glory, not his own. Christ is a lowly door, and he who enters by this door must be lowly, if he would enter with his head whole.”
Now that we have a description of the robber, see that of the Shepherd: “But he that entereth in by the door is the shepherd of the sheep” (v. 2). Christ the Truth is this door, says St. Augustine. And “To him the porter openeth; and the sheep hear his voice: and he calleth his own sheep by name, and leadeth them out” (v. 3).
Who is the porter? St. John Chrysostom suggests that it may be Moses, “for to him the oracles of God were committed.” Theophylact suggests that it may be the Holy Spirit, “by whom the Scriptures are unlocked, and reveal the truth to us.” St. Augustine suggests that the porter is Christ Himself, and for this reason: “There is much less difference between a door and a porter, than between a door and a shepherd. And He has called Himself both the door and the shepherd. Why then not the door and the porter? He opens Himself, that is, reveals Himself. . . . The door is Christ, the Truth. Who openeth the door, but He that will guide you into all Truth?”
“And when he putteth forth his own sheep, he goeth before them, and the sheep follow him: for they know his voice” (v. 4). St. Augustine writes: “Who is this that goeth before the sheep, but He who ‘being raised from the dead, dieth no more’ (Rom 6:9); and who said, ‘Father, I will also that they, whom Thou hast given Me, be with Me where I am’ (Jn 17:24)?”
Quotations from St. Thomas Aquinas, Catena Aurea: Commentary on the Four Gospels Collected Out of the Works of the Fathers, Vol. IV, Part I (Oxford: John Henry Parker, 1845).