Note the use of the masculine pronoun “he” is in direct juxtaposition to the neutral noun “the Spirit of truth”. The Holy Spirit is a Person.

Christ is the Way and the Truth (14:6) and the Holy Spirit is the Guide who shows the way to the Truth (16:14). His guidance of us into the truth is gradual and gentle. We are still learning the truth in Christ and will be for the rest of our lives. Nobody knows all there is to know about Christ. This is due to the fact that God’s supreme revelation is in Christ Jesus, and Jesus is not a concept, but a living Person. In Him, God’s revelation continues on. Note that the Spirit also guides us in our walk in the truth (Rom 8:14).

John 14:6  Jesus answered, “I am the way and the truth and the life. No-one comes to the Father except through me.

John 16:14  He will bring glory to me by taking from what is mine and making it known to you.

Romans 8:14  because those who are led by the Spirit of God are sons of God.

He will guide into all truth and He is certainly in a position to do so (1 Cor 2:10-11). Note also that God does not reveal just theological truth, but all truth. An example is Handel’s explanation of how he wrote the Hallelujah Chorus. His inspiration was that he saw heaven opened and God sitting on His throne. Has God ever revealed any non-theological truth to you? Barclay comments that “Truth is not men’s discovery; it is God’s gift. It is not something which we create; it is something already waiting to be discovered. At the back of all truth there is God.”

1 Corinthians 2:10-11  but God has revealed it to us by his Spirit. The Spirit searches all things, even the deep things of God.  For who among men knows the thoughts of a man except the man’s spirit within him? In the same way no-one knows the thoughts of God except the Spirit of God.

“he will speak only what he hears” – He is not going to teach them or us something brand new, but only what the Father and the Son have already taught. The three Persons of the Godhead are in one accord on what will be said to man. In (1 John 5:7-8) it says that there are three that testify — the Spirit the water and the blood, but it also holds true that there are another three that testify — the Spirit, the Father, and the Son.

1 John 5:7-8  For there are three that testify:  the Spirit, the water and the blood; and the three are in agreement.

“The things to come” for the most part must have been the things already begun concerning the Kingdom (Luke 7:19; 18:29-30). However, the Spirit was going to be a Spirit of prophecy to the apostles (and us) (Joel 2:28) and He most certainly did show them things to come (Acts 11:28; 20:23; 21:11, Rev 1:1-2, 10).

Luke 7:19  he sent them to the Lord to ask, “Are you the one who was to come, or should we expect someone else?”

Luke 18:29-30  “I tell you the truth,” Jesus said to them, “no-one who has left home or wife or brothers or parents or children for the sake of the kingdom of God  will fail to receive many times as much in this age and, in the age to come, eternal life.”

Joel 2:28  “And afterwards, I will pour out my Spirit on all people. Your sons and daughters will prophesy, your old men will dream dreams, your young men will see visions.

Acts 11:28  One of them, named Agabus, stood up and through the Spirit predicted that a severe famine would spread over the entire Roman world. (This happened during the reign of Claudius.)

Acts 20:23  I only know that in every city the Holy Spirit warns me that prison and hardships are facing me.

Acts 21:11  Coming over to us, he took Paul’s belt, tied his own hands and feet with it and said, “The Holy Spirit says, ‘In this way the Jews of Jerusalem will bind the owner of this belt and will hand him over to the Gentiles.’”

Revelation 1:1-2  The revelation of Jesus Christ, which God gave him to show his servants what must soon take place. He made it known by sending his angel to his servant John,  who testifies to everything he saw—that is, the word of God and the testimony of Jesus Christ.

Revelation 1:10  On the Lord’s Day I was in the Spirit, and I heard behind me a loud voice like a trumpet,

Note that those who bypass the teachings of the apostles and regard the gospels as their entire Bible are missing out on the whole truth, as is shown here by Jesus Himself.