Unity in Christ
[Jesus says in John 13:35 that] if an individual Christian does not show love toward other true Christians, the world has a right to judge that he or she is not a Christian. Here [in John 17:21] Jesus is stating something else that is much more cutting, much more profound: We cannot expect the world to believe that the Father sent the Son, that Jesus’s claims are true, and that Christianity is true, unless the world sees some reality of the oneness of true Christians.
– Francis Schaeffer