5 What means it then, that to good shepherds Thou dost set forth One Only Shepherd, but that in One Shepherd Thou teachest unity? And the Lord Himself explains this more clearly by my ministry, putting you, beloved, in remembrance by this Gospel, and saying, “Hear what I have set forth; I have said, 'I am the good Shepherd.' because all the rest, all the good shepherds, are My members.” One Head, One Body, One Christ. So then both the Shepherd of shepherds, and the shepherds of the Shepherd, and the sheep with their shepherds under The Shepherd.
What is all this, but what the Apostle says? “For as the body is one, and has many members, and all the members of the body, being many, are one body; so also is Christ.” Therefore if Christ be even so, with good reason does Christ in Himself containing all good shepherds, set forth One, saying, “'I am the good Shepherd.' 'I am,' I Alone am, all the rest with Me are one in unity. Whoso feeds without Me, feeds against Me. 'He that gathers not with Me, scatters.'” Hear then this unity more forcibly set forth; “Other sheep,” says He, “I have which are not of this fold.” For He was speaking to the first fold of the stock of the fleshly Israel.
But there were others of the stock of the faith of this Israel, and they were yet without, were among the Gentiles, predestinated, not yet gathered in. These He knew who had predestinated them; He knew, who had come to redeem them with the shedding of His Own Blood. He saw them who did not yet see Him; He knew them who yet believed not on Him. “Other sheep,” says He, “I have which are not of this fold;” because they are not of the stock of the flesh of Israel. But nevertheless they shall not be outside of this fold, “for them also I must bring, that there may be One Fold, and One Shepherd.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)