In verse 25 Jesus reminded the disciples then, and us today that hatred of Jesus Christ is not justified. Why would someone hate the One Who promises to forgive sins and grant peace of heart and mind as well as eternal life? Yet that is the history of humanity. All people have hated Jesus Christ at some point in their lives. Some respond to God’s drawing as children while others wait into adulthood to respond and be saved. Most never turn to God and die hating the Saviour Who offers them salvation.
When we look at prophecies like verse 25 we need to understand the wisdom of God. God has not pre-programmed man to be a rebel. God shows us throughout the Bible that He knows our hearts better than we do. If a person rejects Him, they demonstrate their hatred for Him.
In Canada we have an ever expanding list of actions that the government defines as hatred. Many of them are not hatred at all. They are actually acts of love. If we want to understand hatred, we need to look to God’s definition. God shows us that it is completely irrational to hate Him. When we talk of God here we speak of the Godhead. The Jews expressed hatred toward Jesus Christ but claimed to love God the Father. They proved their hatred of God the Father in rejecting Jesus Christ as the Messiah.
In verse 26 Jesus gave the promise that the saints are not left without a Comforter. Jesus identified this Comforter as the Spirit of truth. It is important to note that the Holy Spirit is the Spirt of truth. He would be sent by Jesus Christ from the Father. Here we see again the Godhead revealed. God the Father, God the Son and God the Holy Spirit. These three are One.
We see as well here that the Comforter would testify of Jesus Christ. The Holy Spirit does not go off and do His own thing. The Charismatics and many others who claim to be led by the Holy Spirit into their unbiblical ways cannot legitimately justify their actions or their beliefs. The Holy Spirit will never lead anyone contrary to the written Word of God. God is not adding to the Scriptures. He has warned those who would add or take away from His Word.
In verse 27 we are reminded that not only would the Holy Spirit testify of Jesus Christ but the disciples would also bear witness of Him. Jesus shows us here that He is speaking to the disciples particularly. They had been with Him from the beginning. Jesus chose His disciples early on in His public ministry. He tells us here that they would bear witness of His authenticity and His Words. Luke wrote in the opening verses of his record of the Gospel of Jesus Christ that he was writing as an eyewitness of the truth. Luke was directed to write this:

Luke 1:3 It seemed good to me also, having had perfect understanding of all things from the very first, to write unto thee in order, most excellent Theophilus,
4 That thou mightest know the certainty of those things, wherein thou hast been instructed.

John was directed by God to testify directly of Jesus Christ in I John.

I John 1:1 That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked upon, and our hands have handled, of the Word of life;
2 (For the life was manifested, and we have seen it, and bear witness, and shew unto you that eternal life, which was with the Father, and was manifested unto us;)
3 That which we have seen and heard declare we unto you, that ye also may have fellowship with us: and truly our fellowship is with the Father, and with his Son Jesus Christ.

Peter also testified of Him in II Peter 1:16-18.

II Peter 1:16 For we have not followed cunningly devised fables, when we made known unto you the power and coming of our Lord Jesus Christ, but were eyewitnesses of his majesty.
17 For he received from God the Father honour and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased.
18 And this voice which came from heaven we heard, when we were with him in the holy mount.