What is the context?
Jesus speaks of His joy being fulfilled in the disciples even as they face the world’s hatred. He has given them the Father’s word, and their life is now shaped by belonging to Him. He asks that they be kept from the evil one while remaining in the world.
He then asks the Father to sanctify them by the truth and identifies the Father’s word as truth. The word sanctify is ἁγιάζω, hagiazō (hah-gee-AH-zoh). Thayer’s lexicon explains its senses of dedicating someone to God and making holy, with direct references to verses 17 and 19. Here their dedication and holiness belong to the mission Jesus is giving them.
Jesus says that He sanctifies Himself for their sake. This does not describe His becoming free of a sin He previously had. The same lexical entry identifies His self-dedication through death in verse 19. He is committing Himself to the Father’s work so that His followers may be set apart in truth.
The movement is outward: as the Father sent Him, He sends them into the world. Their holiness is not a reason to abandon the people they are sent to reach. It is part of being equipped to represent Him faithfully.
How is this passage commonly understood?
Christians commonly understand the prayer as asking for protection and holiness within the disciples’ mission. Wesley on verses 15–19 joins their dedication to service with growth in holiness and understands Jesus’ self-dedication in relation to His sacrifice. Calvin on verses 17–19 likewise connects their renewal through the word with Christ’s dedication of Himself for them. The central point is shared: God’s truth shapes the people He sends.
Where does this fit in God's plan?
The connection between Jesus’ self-offering and His people’s holiness is explicit in Hebrews 10:5–10. He comes to do God’s will, and His people are sanctified through the offering of His body. Their dedication rests on His saving work.
The sending continues after the resurrection. In John 20:21–22, Jesus again speaks of sending the disciples as the Father sent Him and gives them the Holy Spirit. The prayer prepares people who will make Him known in the world.
What is our position?
Our study What Does Prayer Look Like? draws attention to Jesus’ requests for His followers to be kept from the evil one and made holy in truth. It takes these as a pattern for interceding for people: seek their faithful formation and life with God, rather than making the removal of hardship the main purpose of prayer. Jesus’ request here concerns people who will remain in the world to serve Him.