Christ said, “Verily, verily, I say unto you, I am the door of the sheep” (Jn 10:7), and “I am the good shepherd: the good shepherd giveth his life for the sheep” (v. 11). Since He is both the Door and the Good Shepherd, how does He enter through Himself? St. Augustine answers: “Just as He knows the Father by Himself, and we by Him; so He enters into the fold by Himself, and we by Him. We enter by the door, because we preach Christ; Christ preaches Himself. A light shows both other things, and itself too. There is but one Shepherd. For though the rulers of the Church, those who are her sons, and not hirelings, are shepherds, they are all members of that one Shepherd. His office of Shepherd He hath permitted His members to bear. Peter is a shepherd, and all the other Apostles: all good Bishops are shepherds. But none of us calleth himself the door.”
Christ then gives the difference between the shepherd and the hireling: “But he that is an hireling, and not the shepherd, whose own the sheep are not, seeth the wolf coming, and leaveth the sheep, and fleeth: and the wolf catcheth them, and scattereth the sheep” (v. 12). Pope St. Gregory the Great said in a homily: “Some there are who love earthly possessions more than the sheep, and do not deserve the name of a shepherd. . . . An hireling is he who holds the place of shepherd, but seeketh not the gain of souls, who panteth after the good things of earth, and rejoices in the pride of station.”
St. Gregory explains how the hireling shows his colors: “Whether a man be a shepherd or an hireling, cannot be told for certain, except in a time of trial. In tranquil times, the hireling generally stands watch like the shepherd. But when the wolf comes, then every one shows with what spirit he stood watch over the flock. . . . The wolf too cometh upon the sheep, whenever any spoiler and unjust person oppresses the humble believers. And he who seems to be shepherd, but leaves the sheep and flees, is he who dares not to resist his violence, from fear of danger to himself. He flees not by changing place, but by withholding consolation from his flock. The hireling is inflamed with no zeal against this injustice. He only looks to outward comforts, and overlooks the internal suffering of his flock.”
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).