Please note the pronouns in verses 17 and 18. Verse 17 says that "He" shall be in you, while verse 18 says that "I" am coming to you. What is the relationship between the He in verse 17 and the I in verse 18? Do they qualify as two persons or one? It is best for us to read the two verses again to find out who the "He" and the "I" are in verses 17 and 18.
If I were to tell you something like this, "A man just stopped a taxi by the city hall. He paid five dollars and had the driver bring him here. When I arrived, I ran in and saw all of you sitting here, and I began to preach." Please tell me what would be the relationship between that man and me? Of course, that man is I and I am that man. It is just a different way of mentioning the same person. Likewise the "I" in verse 18 is the same one as the "He" in verse 17. The meaning of this passage is clear; the Lord will ask the Father, and the Father will send a Comforter to your midst. This Comforter, who is the Holy Spirit, is just the Christ that will dwell inside of you. In this way the disciples will no longer remain orphans.
I IN HIM
This is a most precious portion of the Bible. When Christ was on the earth, the Holy Spirit, who is the Comforter, lived inside of Him. After His death, resurrection, and ascension, He lives inside the Spirit. When Christ was with the disciples on earth, the Holy Spirit was with the disciples already, for the Spirit was inside Christ. But what happened after those days? By His death and resurrection, Christ is now in the Spirit. Hence, He is coming to His disciples by the Spirit, through the Spirit, and in the Spirit. You may say that the Holy Spirit is now in the disciples. But actually it is Christ in the disciples. This is why in the first part of this portion it says, "He...shall be in you," but then the latter part changes to "I am coming to you." The "I" is in the "He."
THE HAND IN THE GLOVE
One time I was talking to a friend of mine in Kaifeng concerning Christ’s being in the Holy Spirit. A foreign lady happened to come by. She greeted me and was about to take off her gloves to shake my hand. I rushed up to her and said, "There is no need to take off your gloves." I held up the hand with the glove and turned to my friend, asking, "Am I holding the glove or the hand? You may say that I am holding her glove, but I am actually holding her hand. The relationship between Christ and the Holy Spirit is exactly the same. When you outwardly take hold of the Spirit, you are actually possessing Christ inwardly. Receiving the Holy Spirit is the same as receiving Christ." My friend nodded his head and said, "Now I see!"
This is similar to what we said earlier about the divinity of Jesus of Nazareth. You may say that He is God and also say that He is a man. When you touch His divinity, you cannot avoid touching His humanity also. When His humanity is manifested, His divinity simultaneously comes forth. In the same way, Christ and the Spirit are one inseparable entity. You may interchange the two terms as synonyms. The Spirit is in Him, and He is in the Spirit.
(The Normal Christian Faith, Chapter 11, by Watchman Nee)