Let us explain the parable. Christians are the sheep of Christ's pasture. The Shepherd whom they must hear is, originally and essentially, Jesus Christ Himself. After Him, they must also hear whoever visibly represent His invisible shepherding and act by the authority entrusted to them by Him.
For, as the Apostle says, He gave the Church "pastors and teachers, for the perfecting of the saints, for the work of ministry" (Eph 4:11-12). In order that those given by Him might be recognized, He set this sign: "He who enters by the door is the shepherd of the sheep" (Jn 10:2). That is, one who enters this ministry through gracious election.
And so that these ministering shepherds might be heard as the supreme Shepherd Himself, He proclaimed that to hear them and to hear His words are one: "He who hears you hears Me" (Lk 10:16).
The sheepfold and spiritual pasture, where we hear the Shepherd's voice and where the sheep find food and salvation, is the Church. The Shepherd's voice is prayer, teaching, sacramental action, governance, and counsel profitable for the soul.
If people who call themselves Christians leave the Church and wander through the wilds of schisms, "tossed to and fro and carried about with every wind of doctrine," by human deceit and cunning craftiness (Eph 4:14), what then?
If instead of the true shepherd, who enters by the door, they listen to those who climb in another way (Jn 10:1), then, according to the judgment of the Chief Shepherd, these are thieves and robbers.
Or if, even without visibly separating from the Church, they do not cleave to her with a sincere and obedient heart, what then? If they neglect to hear the shepherd when he prays, teaches, and performs the mysteries, and if they do not seek spiritual counsel, what then?
If through ignorance and pride they imagine that they can shepherd and save themselves, such people, and those like them, are not true sheep, because they do not hear the true Shepherd.
By turning aside from obedience to the true and saving voice of the Chief Shepherd, they imperceptibly go to meet the lion who "walks about roaring, seeking whom he may devour" (1 Pet 5:8).