"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."
"Truth" is a major theme in John's Gospel. In fact, it will be Pontius Pilate who asks the question on the world's mind as they are confronted with Jesus: "What is truth?"
The truth is, however, that Jesus himself is the way, the truth and the life. In other words, if we emulate Jesus, our lives will not be a lie. Our lives instead will be lives of integrity, honesty, service, selflessness and humility -- all counterintuitive from the world's point of view.
If the disciples want to follow Jesus where he is going, then that means following his way, his truth and his life in spite of the dangers, twists and turns ahead. The only way to navigate that way is with a Guide who will take what Jesus has said and done and "declare it" to us so that we will glorify him