Fundamental to any understanding of biblical unity is Jesus’ prayer in John 17. Jesus is about to return to the Father and leave His followers behind in a hostile world. His prayer is for their continued safety and unity (John 17:11-12, 15). What “sanctifies” them—what sets them apart for God—is the “word” of God.
I have given them [the apostolic band] the words that You [Father] gave Me [Jesus], and they have received them. … I have given them Your word, and the world has hated them because they are not of the world, just as I am not of the world. … Sanctify them in the truth; Your word is truth (John 17:8, 14, 17).
Having prayed for the apostolic band, Jesus then focuses His prayer on future believers. Again, the “word” plays a crucial role.
I do not ask for these only, but also for those who will believe in Me through their word, that they may all be one … (John 17:20-21).
“Their word” comes from the Father through the Son and the Holy Spirit, as promised in the same context: “When he Spirit of truth comes, He will guide you into all the truth” (John 16:13). Thus, Jesus prays for the unity of genuine disciples, those who respond to the apostles’ teaching. By Jesus’ own definition, a disciple is a believer who “abides” in His Word (John 8:31 cf. John 15:3-4, 7; Acts 2:42). The fact that Jesus has in mind a specific group genuine disciples who abide in His Word—makes all the difference to how we understand unity. This becomes clearer when we realize for whom Jesus is not praying.
Christ’s disciples “abide” in His Word.