1 Those of you who hear the word of our God, not only with willingness, but also with attention, doubtless remember our promise. Indeed the same gospel lesson has also been read today which was read last Lord's day; because, having lingered over certain closely related topics, we could not discuss all that we owed to your powers of understanding. Accordingly, what has been already said and discoursed about we do not inquire into to day, lest by continual repetitions we should be prevented from reaching what has still to be spoken.
You know now in the Lord's name who is the good Shepherd, and in what way good shepherds are His members, and therefore the Shepherd is one. You know who is the hireling we have to bear with; who the wolf, and the thieves, and the robbers we have to beware of; who are the sheep, and what is the door whereby both sheep and shepherd enter: how we are to understand the doorkeeper. You know also that every one who enters not by the door is a thief and a robber, and comes not but to steal, and to kill, and to destroy.
All these sayings have, as I think, been sufficiently handled. Today we ought to tell you, as far as the Lord enables us (for Jesus Christ our Saviour has Himself told us that He is both the Shepherd and the door, and that the good Shepherd enters in by the door), how it is that He enters in by Himself. For if no one is a good shepherd but he that enters by the door, and He Himself is preeminently the good Shepherd, and also Himself the door, I can understand it only in this way, that He enters in by Himself to His sheep, and calls them to follow Him, and they, going in and out, find pasture, which is to say, eternal life.
Source: Tractates on the Gospel of John (New Advent)