Augustine of Hippo — as excerpted in the Catena Aurea on John 10:1-5

“(Tr. xlvi. 2) Or, the porter is our Lord Himself; for there is much less difference between a door and a porter, than between a door and a shepherd. And He has called Himself both the door and the shepherd. Why then not the door and the porter? He opens Himself, i. e. reveals1 Himself. If thou seek another person for porter, take the Holy Spirit, of whom our Lord below saith, He will guide you into all truth. (c. 16:13) The door is Christ, the Truth; who openeth the door, but He that will guide you into all Truth? Whomsoever thou understand here, beware that thou esteem not the porter greater than the door; for in our houses the porter ranks above the door, not the door above the porter.”