Thomas Manton, a seventeenth century minister, once wrote, "Divisions in the church breed atheism in the world." Certainly the lack of unity in the church distracts minds, breaks hearts, squanders energy, and inhibits evangelism. Unity in the church is important to God. John 17 has been described as “a standing monument of Christ’s affection to the Church." At least three times Jesus prays for the Church’s unity and witness: "that they all may be one" (v. 21); "that they may be one even as we are one" (v. 22); "that they may become perfectly one" (v. 23); so that all "the world may believe that you have sent me... and loved them even as you loved me" (vv. 21, 23). So the stakes are high. In the twentieth century, these verses were taken out of context and used to argue for a lowest-common-denominator kind of unity—an institutional union that flattened out distinctions and minimized the very doctrines that make the church distinctively "Christian." What is striking, however, is to see that, rather than being minimalistic, Jesus’ prayer paints a grand picture of the rich contours of Christian belief that hold His people together in the world.
Specifically, the church is united in a shared history. Here we find an exalted view of God the Trinity as the Son speaks to His Father of the pretemporal glory they shared “before the world existed” (v. 5). We listen to the Son speak of an eternal covenant, or arrangement, forged between the members of the Godhead in which they planned the salvation of a people out of the world. (Theologians call this the pactum salutis, or the covenant of redemption.) In other words, the church’s history began before history in the mind and heart of God, when the Father promised a people as a love gift to His Son. We hear the Son reporting that He had "accomplished the work" that the Father had given Him to do. That work finds its focus on the cross, for "the hour has come" when both God the Father and the Son will be "glorified" (17:1). He had linked His glory to His death before when He said, "The hour has come for the Son of Man to be glorified.... Unless a grain of wheat falls into the earth and dies, it remains alone; but if it dies, it bears much fruit" (12:23–24). This was the "hour" for Him to "depart out of this world to the Father" (13:1). The hour of His death would be the hour of His glorification. It is with a view to this death that the Son, as our Great High Priest, "consecrates" Himself (17:19) the sacrificial victim to be our sinbearer and Savior. So we have a shared history as those chosen by the Father, then given to and redeemed by the Son.
The church is united in a shared legacy, which was given by Christ to the Apostles for our sake. John 17:6–17 refers primarily to these men whom Christ had chosen and gathered around Him in the upper room. He had "manifested" the Father’s “name” to them (17:6) and given them God’s “words” (17:8). That “word” of God, given to the Apostles, "is truth" (17:16), and it is "through their word" that we today have come to "believe" in Jesus (17:20). In other words, as our Lord peers into the future, He sees generation after generation of His followers who will believe in Him through the word of the Apostles.
From the very beginning, the church of Christ has been an Apostolic church: "and they devoted themselves to the apostles’ teaching and the fellowship" (Acts 2:42). This leads the Apostle Paul to say that the church is one, a single building, God’s temple, because it is “built on the foundation of the apostles and prophets" (Eph. 2:20). For Jesus, it is the truth that unites us. Our spiritual legacy is both a common truth and a common life, for the church shares the very life of God by being organically united to both the Father and the Son: "just as you, Father, are in me, and I in you, that they also may be in us" (17:21). John Stott has written,
The two potential enemies of Christian unity are time and space. It is these that separate believers from each other. But the same apostolic truth spans the successive generations of the church, and the same divine life animates all believers of the same generation.
The church is united in a shared destiny. For as Jesus finishes His prayer, He looks beyond history to eternity and expresses His final will for His church. He prays that His people will be with Him where He is and see His glory. This is the church’s destiny: both to be with Christ and to see Him as He is, and the vision of Jesus will be the vision of God. What distinguishes the church from the world now is that the world does not know God, but Christ has made Him known to the church. In eternity, that knowledge will be complete, and our fellowship perfect, for we will enjoy the very same love with which the Father has loved the Son (17:26).