What does John 15:26 mean?

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.