2 We understand the Lord Christ as the door, and also as the Shepherd; but who is to be understood as the doorkeeper? For the former two, He has Himself explained: the doorkeeper He has left us to search out for ourselves. And what does He say of the doorkeeper? “To him,” He says, “the porter [doorkeeper] opens.” To whom does he open? To the Shepherd. What does he open to the Shepherd? The door. And who is also the door? The Shepherd Himself. Now, if Christ the Lord had not Himself explained, had not Himself said, “I am the Shepherd,” and “I am the door,” would any of us have ventured to say that Christ is Himself both the Shepherd and the door?
For had He said, “I am the Shepherd,” and had not said, “I am the door,” we should be setting ourselves to inquire what was the door, and perhaps, mistaken in our views, be still standing before the door. His grace and mercy have revealed to us the Shepherd, by His calling Himself so; have revealed to us also the door, when declared Himself such; but He has left us to search out the doorkeeper for ourselves. Whom, then, are we to call the doorkeeper? Whomsoever we fix upon, we must take care not to think of him as greater than the door itself; for in men's houses the doorkeeper is greater than the door.
The doorkeeper is placed before the door, not the door before the doorkeeper; because the porter keeps the door, not the door the porter. I dare not say that any one is greater than the door, for I have heard already what is the door: that is no longer unknown to me, I am not left to my own conjecture, and I have not got much room for mere human guess work: God has said it, the Truth has said it, and we cannot change what the Unchangeable has uttered.
Source: Tractates on the Gospel of John (New Advent)