Ian asked a good question at the Bible Study last night: "If we are already one because we are connected to Jesus then how can he pray for us to be one in John 17?"
Carson says: “Although the unity envisaged in this chapter is not institutional, this purpose clause at the end of v. 21 shows beyond possibility of doubt that the unity is meant to be observable. It is not achieved by hunting enthusiastically for the lowest common theological denominator, but by common adherence to the apostolic gospel, by love that is joyfully self-sacrificing, by undaunted commitment to the shared goals of the mission with which Jesus' followers have been charged, by self- conscious dependence on God himself for life and fruitfulness. It is a unity necessarily present, at least in nuce, amongst genuine believers; it is a unity that must be brought to perfection (v. 23).”