In John 13 the point was that, if an individual Christian does not show love toward other true Christians, the world has a right to judge that he is not a Christian. Here (in John 17:21) Jesus is stating something else which is much more cutting, much more profound: We cannot expect the world to believe the Father sent the Son, that Jesus' claims are true, and that Christianity is true, unless the world sees some reality of the oneness of true Christians.