John 17: What to preach this Reformation season
What about John 17 and Christian unity?
Raymond E. Brown, renown Roman Catholic New Testament scholar and specialist in the “Johannine community,” helps with John 17:21-22: 
“…that they may all be one; even as thou, Father, art in me, and I in thee, that they also may be in us, so that the world may believe that thou hast sent me. The glory which thou hast given me I have given to them, that they may be one even as we are one.”
Brown notes that these verses should not be interpreted in an overly ecumenical way. They are not about elevating unity to an absolute principle. A little “god” claiming preeminence. Unity can be wrongly used as a pretext to censor those, like Luther, who challenge the church about gospel truth. It’s an old problem. Brown notes how 2 John 10 warns Christians not to welcome other Christians who have adopted false teachings. Truth trumps unity.
Brown identifies a vertical and horizontal dimension to John 17 (11, 21-23). First, the vertical: Christ gives believers the glory that the Father has given to him. Glory comes down from the Father and Son to believers. It is God’s action, God’s doing.
To be sure, it is all God’s doing: “You did not choose me, but I chose you” (John 15:16). In baptism the Lord comes down and snatches us from the Evil One. The day we are baptized is our first day of eternal life. The happy exchange happens to us. Glory, alleluia!
Second, the horizontal: In the gospel of John faith comes by hearing about who Jesus is, that is, about the specific and intimate relation between the Father and the Son. “No one has ever seen God; the only Son, who is in the bosom of the Father, he has made him known” (John 1:18). He was crucified, and the Father vindicated him, establishing his authority as the Son. Who is God? The Father of our Lord, Jesus Christ. John 17 focuses on this: Preach Christ crucified and vindicated, “so that the world may believe that Thou hast sent me” (John 17:21).
The horizontal, like the vertical, is about gospel truth: The Father gives life to the Son (“thou, Father, art in me, and I in thee”). Christians are one with each other and with the Father and the Son because they have received this life in baptism and for eternity.
John 17 isn’t about church mergers and union agreements. It isn’t about ecumenical programs to achieve uniformity. After all, churches can be so compromised by other agendas that they no longer can effectively preach the gospel. When this happens, like Luther, we must protest – reform them, or leave them, or risk compromising the truth of the gospel ourselves.
John 17 is about Christian unity. Its message: Only the truth itself, Jesus Christ, establishes unity. The unity of churches cannot be achieved by the churches moving toward each other, but by each church moving towards the truth in order to let itself there, in the truth, be united with the others.
 Raymond E. Brown, The Gospel According to John XIII-XXI, Anchor Bible Series (Garden City: Doubleday, 1970) 768-79.