Jesus was giving His trainee apostles their final briefing before He went to the cross.  He had just told them that the sign of their love for Him, would be their obedience to His teaching (John 14:15; John 14:21).  Others, like Judas Iscariot who had no devoted love for Him, would prove it by their disobedience (John 14:24).  But there was so much that Jesus had said, how could they remember?  And there were more important instructions which they could not absorb; how would Jesus communicate all that they needed to know to be effective witnesses, and to lead the Church after Jesus had returned to heaven?   

They needed Jesus’ reassurance that they would not be left alone (John 14:18).  He reassured them that the Holy Spirit’s presence would be just like having Him alongside each of them all the time.  He would not only comfort them but also be their Advocate – giving them the words to say when they were wrongly accused (Matthew 10:19-20).  The Spirit would bring back to their minds what Jesus had already said, and provide them with more instruction about how to put the teaching into practice.  
 
God had already planned for His Holy Spirit to live inside everybody who believes in Jesus (Romans 8:9).  He would be a teacher like Jesus.  That is why the gospel narratives have so much detail, as though a reporter was there at the time writing it all down.  When the time came for the gospels to be recorded, the Holy Spirit gave the writers a vividly accurate recollection of what Jesus said and did (Acts 1:1-2).
 
The whole Bible was written under the Holy Spirit’s inspiration (2 Peter 1:20-21).  As He also indwells every believer, He helps us to understand what God is saying when we read the Scriptures.  Because He is unchanging, the Spirit’s teaching is consistent with the words of Scripture and the recorded words of Jesus.  So, these verses give us great confidence that God will speak to us through the Holy Spirit, as though Jesus is in the room with us.  That gives us all the more reason to invest our time and energy in reading God’s Word and asking the Holy Spirit to help us understand it.  So, as we read these verses today, it is right to ask God to speak to us through His Holy Spirit and show us whatever He wants to teach us.