“Howbeit when he, the Spirit of truth, is come, he will guide you into all truth: for he shall not speak of himself; but whatsoever he shall hear, that shall he speak: and he will shew you things to come.”
When the Spirit of truth comes, he will guide you into all the truth; for he will not speak on his own, but will speak whatever he hears, and he will declare to you the things that are to come — the Spirit's function is to lead progressively into truth (hodos—way, path; guiding into all truth is a journey, not instantaneous). The Spirit does not originate new doctrines but speaks what he hears from the Father; he is a faithful interpreter of the Word Jesus has spoken. The Spirit guides the disciples not into abstract truth but into living understanding of Jesus Christ, the Truth incarnate. The promise that the Spirit will declare things to come (ta erchomena—the things coming) suggests prophetic illumination and interpretation of Scripture.
Church Fathers on John 16:13
Our Lord having said above, It is expedient for you that I go away, He enlarges now upon it: I have yet many things to say to you, but you cannot bear them now.
All heretics, when their fables are rejected for their extravagance by the common sense of mankind, try to defend themselves by this text; as if these were the things which the disciples could not at this time bear, or as if the Holy Spirit could teach things, which even the unclean spirit is ashamed openly to teach and preach. But bad doctrines such as even natural shame cannot bear are one thing, good doctrines such as our poor natural understanding cannot bear are another.…
Having said then, you cannot bear them now, but then you shall be able, and, The Holy Spirit shall lead you into all truth; lest this should make them suppose that the Holy Spirit was the superior, He adds, For He shall not speak of Himself, but whatsoever He shall hear, that shall He speak.