3 In respect, then, of the profound nature of this question, I shall tell you what I think: let each one make the choice that pleases him, but let him think of it reverently; as it is written, “Think of the Lord with goodness, and in simplicity of heart seek Him.” Perhaps we ought to understand the Lord Himself as the doorkeeper: for the shepherd and the door are in human respects as much different from each other as the doorkeeper and the door; and yet the Lord has called Himself both the Shepherd and the door.
Why, then, may we not understand Him also as the doorkeeper? For if we look at His personal qualities, the Lord Christ is neither a shepherd, in the way we are accustomed to know and to see shepherds; nor is He a door, for no artisan made Him: but if, because of some point of similarity, He is both the door and the Shepherd, I venture to say, He is also a sheep. True, the sheep is under the shepherd; yet He is both the Shepherd and a sheep. Where is He the Shepherd? Look, here you have it; read the Gospel: “I am the good Shepherd.”
Where is He a sheep? Ask the prophet: “He was led as a sheep to the slaughter.” Ask the friend of the bridegroom: “Behold the Lamb of God, that takes away the sin of the world.” Moreover, I am going to say something of a still more wonderful kind, in accordance with these points of similarity. For both the lamb, and the sheep, and the shepherd are friendly with one another, but from the lions as their foes the sheep are protected by their shepherds: and yet of Christ, who is both sheep and Shepherd, we have it said, “The Lion of the tribe of Judah has prevailed.” All this, brethren, understand in connection with points of similarity, not with personal qualities.
It is a common thing to see the shepherds sitting on a rock, and there guarding the cattle committed to their care. Surely the shepherd is better than the rock that he sits upon; and yet Christ is both the Shepherd and the rock. All this by way of comparison. But if you ask me for His peculiar personal quality: “In the beginning was the Word, and the Word was with God, and the Word was God.” If you ask me for the personal quality peculiarly His own: The only Son, from everlasting to everlasting begotten of the Father, the equal of Him that begot, the Maker of all things, unchangeable with the Father, unchanged by the assuming of human form, man by incarnation, the Son of man, and the Son of God. All this that I have said is not figure, but reality.
Source: Tractates on the Gospel of John (New Advent)