"I am the Good Shepherd." Surely it is fitting that Christ should be a shepherd, for just as a flock is guided and fed by a shepherd so the faithful are fed by Christ with spiritual food and with his own body and blood. The Apostle said: "You were once like sheep without a shepherd, but now you have returned to the guardian and ruler of your souls." The prophet has said: "As a shepherd he pastures his flock."
Christ said that the shepherd enters through the gate and that he is himself the gate as well as the shepherd. Then it is necessary that he enter through himself. By so doing, he reveals himself, and through himself he knows the Father. But we enter through him because through him we find happiness.
Take heed: no one else is the gate but Christ. Others reflect his light, but no one else is the true light. John the Baptist "was not the light, but he bore witness to the light." It is said of Christ, however: "He was the true light that enlightens every man." For this reason no one says that he is the gate; this title is Christ’s own. However, he has made others shepherds and given that office to his members; for Peter was a shepherd, and so were the other apostles and all good bishops after them. Scripture says: "I shall give you shepherds according to my own heart." Although the bishops of the Church, who are her sons, are all shepherds, nevertheless Christ refers only to one person in saying: "I am the Good Shepherd," because he wants to emphasize the virtue of charity. Thus, no one can be a good shepherd unless he is one with Christ in charity. Through this we become members of the true shepherd.
The duty of a good shepherd is charity; therefore Christ said: "The good shepherd gives his life for his sheep." Know the difference between a good and a bad shepherd: the good shepherd cares for the welfare of his flock, but the bad shepherd cares only for his own welfare.
The Good Shepherd does not demand that shepherds lay down their lives for a real flock of sheep. But every spiritual shepherd must endure the loss of his bodily life for the salvation of the flock, since the spiritual good of the flock is more important than the bodily life of the shepherd, when danger threatens the salvation of the flock. This is why the Lord says: "The good shepherd lays down his life," that is, his physical life, "for his sheep;" this he does because of his authority and love. Both, in fact, are required: that they should be ruled by him, and that he should love them. The first without the second is not enough.
Christ stands out for us as the example of this teaching: "If Christ laid down his life for us, so we also ought to lay down our lives for our brothers."
The remnant of Israel shall be led to pasture, They shall lie down and rest
From the Exposition on John by Saint Thomas Aquinas, priest
The Liturgy of the Hours, Vol. IV, Office of Readings
Monday of the 21st Week of Ordinary Time