12 I still have many things to say to you, but you cannot bear them now.
13 However, when He, the Spirit of truth, has come, He will guide you into all truth; for He will not speak on His own authority, but whatever He hears He will speak; and He will tell you things to come.
14 He will glorify Me, for He will take of what is Mine and declare it to you.
15 All things that the Father has are Mine. Therefore I said that He will take of Mine and declare it to you.
cannot bear them (v. 12) — showing the disciples need for the Spirit.
truth (v. 13) = (in Greek) THE Truth — the living Word and the written Word (the Bible). The written Word witnesses of Christ, the living Word and the Truth. The Spirit is a qualified witness to the Truth because He is the Spirit of Truth.
guide (v. 13) = lead the way (to the truth of Christ) into all truth (all truth revealed in the Word)
The Son, when He was on Earth, spoke from the Father. The Spirit speaks from the Son (v. 13).
The Spirit tells what is to come (v. 13) — He explains the events of the next few days (Christ's death and resurrection). This also includes the inspiration given to Paul, Peter, James, John, Jude, all of which include prophecy.The Spirit tells of the past (John 14:26), the present (John 15:26) and the future (John 16:13).
The Spirit carries on Christ's ministry (v. 13). But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all things that I said to you. (John 14:26).
The Spirit glorifies the Son (v. 14).
The Spirit bears witness of Christ (v. 14). But when the Helper comes, whom I shall send to you from the Father, the Spirit of truth who proceeds from the Father, He will testify of Me (John 15:26).
The Spirit, in harmony with the Father and the Son, glorifies the Son. All He says is from the Son (as all the Son says is from the Father).The Spirit speaks of Himself, but His message is from the Father and the Son.
The Spirit speaking for the Son, and the Son speaking for the Father, doesn't indicate inferiority but willing submission based on a unity of purpose.
Verses 8-11 show what the Spirit does in the world. Verses 12-15 show what He does in believers.