John’s point in 1 John 4, “God is love,” is that those who really do know God come to love that way too. Doubtless we do not do it very well, but aren’t Christians supposed to love the unlovable-even our enemies? Because the Gospel has transformed us, our love is to be self-originating, not elicited by the loveliness of the loved. For that is the way it is with God. He loves because love is one of His perfections, in perfect harmony with all His other perfections. ~ D.A. Carson, The Difficult Doctrine of the Love of God, p. 63.
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.~ Francis Schaeffer, The Mark of the Christian
In trying to track down a favorite Schaeffer quite, I cam across an interesting article you might want to glance at:
http://www.wnd.com/index.php?fa=PAGE.view&pageId=59599