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.
I read Francis Schaeffer’s The Mark of the Christian shortly after it was published in 1970. Schaeffer quoted Christ’s words in John 13:35: “By this all people will know that you are my disciples, if you have love for one another.” Then he cited Jesus’s prayer in John 17:21 that the disciples “may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me.”
Schaeffer tied the verses together:
[In John 13:35] 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. (26–27)
A beautiful, biblical slap in the face.