(26) When the Advocate comes, whom I will send to you from the Father – the Spirit of truth who goes out from the Father – he will testify about me.
(27) And you also must testify, for you have been with me from the beginning.
John 15:26–27, NIV
Great God and Savior, you want to lead us by the hand so that in communion with you we learn how to live a true life. We thank you for everything we have already received. Guide us still, we pray. Through your Spirit leads us in all areas of our life. Grant us the Spirit, who can illumine our hearts to help us find new courage and new strength and new recognition of the truth. All our praise belongs to you, for you alone can quicken us. You alone free us from the pain of death and from all burdens, so that in spite of toil and struggle we may always be lifted up to you, our God on high, to the glory of your name on earth. Amen.
The context of the words of John 15:26–27, NIV
John 15:18–27 predicts that those who follow Christ faithfully will experience hatred and persecution from the world. The intensity of this experience has varied according to time and culture, but the non-believing world is generally hostile to authentic faith. Jesus reminds His followers this is due to sin and rejection of God by those unbelievers. Just as Christians are never "too good" to serve as Christ served, they are never "too good" to suffer as Christ suffered. In the following passage, Jesus emphasizes that this warning is meant to bolster faith when those hard times come.
This passage begins with a celebrated analogy from Christ: the vine and the branches. This includes the seventh and final "I Am" statement of the gospel of John. Jesus uses this metaphor to explain how our spiritual life, as born-again believers, is drawn from His life. God's intent for our lives is to progress from barrenness to fruitfulness, to spiritual abundance. Jesus also repeats His command for believers to love each other. In this context, He goes so far as to refer to those who follow His teachings as His "friends."
Meaning of the words
"When the Advocate comes, whom I will send to you from the Father – the Spirit of truth who goes out from the Father – he will testify about me"
Verses 26 and 27 form a preview of Jesus' next major theme: the work of the Holy Spirit. That explanation will begin in earnest in John 16:5. For now, Jesus mentions the Holy Spirit as part of His encouragement in the face of persecution. Over the last several verses, Jesus has been warning the disciples—and, by extension, all believers—that the unbelieving world will hate them for their faith (John 15:18–25). Telling us about this in advance (John 13:19; 14:25, 29; 16:4) is meant to help us "hold fast" (Hebrews 4:14–16; 12:1–3). Instead of panicking, and wondering if Jesus was wrong, we can remember His words and know He is still in control (1 Peter 4:12–13).
Another aspect of comfort that Jesus brings is predicting the influence of the Holy Spirit. Jesus uses the same term He applied in John 14:16, paraklētos, also translated as "helper," "counselor," "comforter," or "advocate." Part of the Holy Spirit's role in helping us endure persecution is reminding us of these teachings from Jesus (John 14:16; 16:13). The Spirit comes from God in exactly the same sense in which Christ comes from God (John 8:42; 14:26).
This brief mention of the Holy Spirit also helps to take some of the sting out what comes next. At the beginning of chapter 16, Jesus will give a much more specific, dire description of what persecution can look like.
"And you also must testify, for you have been with me from the beginning"
Recent verses mostly focused on how the world will hate Christians, specifically because Christians follow the teachings of Jesus (John 15:18–20). That hatred reflects unbelief; it's one of many ways those who hate truth attempt to resist it (John 15:21–25). As Jesus continues, with the verses starting chapter 16, He will return to that theme in more specific detail (John 16:1–4). For now, though, His intent is to preview the work of the Holy Spirit (John 16:5–14).
A major role of the Holy Spirit is to "bear witness" about Christ in the hearts and minds of believers (John 14:16; 15:26). The influence of the Spirit leads us towards the truth, which is Christ (John 14:6). This makes sense, since the Holy Spirit and Jesus Christ are both sent from God the Father (John 8:42; 14:26).
The reference to people being with Jesus "from the beginning" seems specifically aimed at the disciples. Their eyewitness accounts of Christ's teaching began at the very start of His public ministry. Those accounts would not merely inform the apostles as they spread the gospel. Those memories would also serve as the foundation of the written records we have preserved in the four Gospels and the book of Acts (Luke 1:2; Acts 1:21-22), as well as other writings of the New Testament (2 Peter 1:16; 1 John 1:1).
Christians, themselves, also serve as reminders of the teachings of Jesus Christ. The Greek root verb here is martyr󠅍eō, and the noun form is martys. In the literal Greek, this is someone who serves as a witness, such as in a legal setting, or who provides evidence. Over time, the term became associated with those who "gave evidence" of their faith by enduring violence and death: the modern English term is martyr.
That is the theme Jesus introduces in the next verses, as He explains the extent to which God's enemies will assault God's people.