4 Therefore, let us not, brethren, be disturbed in understanding Him, in harmony with certain resemblances, as Himself the door, and also the doorkeeper. For what is the door? The way of entrance. Who is the doorkeeper? He who opens it. Who, then, is He that opens Himself, but He who unveils Himself to sight? See, when the Lord spoke at first of the door, we did not understand: so long as we did not understand, it was shut: He who opened it is Himself the doorkeeper. There is no need, then, of seeking any other meaning, no need; but perhaps there is the desire. If there is so, quit not the path, go not outside of the Trinity. If you are in quest of some other impersonation of the doorkeeper, bethink you of the Holy Spirit; for the Holy Spirit will not think it unmeet to be the doorkeeper, when the Son has thought it meet to be Himself the door. Look at the doorkeeper as perhaps the Holy Spirit: about Him the Lord says to His disciples, “He shall guide you into all truth.” What is the door? Christ. What is Christ? The Truth. Who, then, opens the door, but He who guides into all truth?
Source: Tractates on the Gospel of John (New Advent)