What does John 17:17 mean?

Jesus continues to pray for the apostles (John 17:6), and by extension the believers who will follow them (John 17:20).

The term "sanctify" is translated from the Greek word hagiazō, the same root from which the term "saint" is derived. This means to separate something—to set it apart—for an intended purpose. Leading up to this request, Jesus has referred to these men as being "in" the world (John 17:11), but also not "of" the world (John 17:14). Being "separated," in this case, does not mean a physical distance. It refers to a special purpose and a unique mission. All Christians are called on to be separated from the unbelieving world, spiritually (1 Corinthians 5:9–11) while also active in the world, practically (Matthew 5:13–16).

Earlier this same evening, Jesus indicated the disciples were made "clean" by the message they had accepted (John 15:3; 17:8). That message is the "word" of God: the overall statement of truth He sends. In this exact context, it does not refer only to the written Scripture, though that is one part of God's message to us. The Greek term used here is logos, the same used by John at the beginning of this gospel in reference to Jesus (John 1:1). God's message is personal, in the form of Jesus Christ (2 Corinthians 4:6; Hebrews 1:3) as well as textual, in the form of the Bible (2 Timothy 3:16–17).

Christ equates that message—the "word"—with truth, itself. What God tells us through the person, the teaching, and the message of Christ is that which is real. It is actual; it is "truth" in the deepest and most fundamental sense. When God identified Himself to Moses, He used the expression "I Am" (Exodus 3:14). This is a phrase Jesus also echoed, as recorded seven times in John's gospel (John 6:35; 8:12; 10:7–9; 10:11; 11:25; 14:6; 15:1). God is truth, and salvation comes when we accept the reality of who He is, and who we are (John 4:24; Romans 1:21–25; Philippians 2:9–11).