14 By this, then, which the Lord has explained, that He Himself is the door, let us find entrance to what He has set forth, but not explained. And indeed who it is that is the Shepherd, although He has not told us in the lesson we have read today, yet in that which follows He very plainly tells us: “I am the good Shepherd.” And although He had not said so, whom else but Himself ought we to have understood in those words where He says, “He that enters in by the door is the Shepherd of the sheep.
To Him the porter opens: and the sheep hear His voice: and He calls His own sheep by name, and leads them out. And when He puts forth His own sheep, He goes before them, and the sheep follow Him: for they know His voice”? For who else calls His own sheep by name, and leads them hence unto eternal life, but He who knows the names of those that are fore-ordained? Hence He said to His disciples, “Rejoice that your names are written in heaven;” for from this it is that He calls them by name.
And who else puts them forth, save He who puts away their sins, that, freed from their grievous fetters, they may be able to follow Him? And who has gone before them to the place whither they are to follow Him, but He who, rising from the dead, dies no more; and death shall have no more dominion over Him; and who, when He was manifest here in the flesh, said, “Father, I will that they also whom You have given me be with me where I am”? Hence it is that He says, “I am the door: by me if any man enter in, he shall be saved, and shall go in and out, and find pasture.” In this He clearly shows that not only the Shepherd, but the sheep also enter in by the door.
Source: Tractates on the Gospel of John (New Advent)