That the world may know

Francis Schaeffer is one of the most solid writers I have ever read. While his pieces are pithy and quick, they hammer home and convict the soul. This past Sabbath I spent the time reading The Mark of the Christian. The entire book is a sermon on John 13:33-35 and John 17:21. Christ answering the question what is the greatest commandment, takes a turn and replies, "A new commandment I give to you, that you love one another: just as I have loved you, you also are to love another. By this all people will know that you are my disciples, if you love one another."

Schaeffer comments:

The Church is to be a loving church to a dying culture. How then, is the dying culture to consider us? In the midst of the world, in the midst of a dying culture, Jesus is giving a right to the culture. Upon his authority he gives the world the right to judge whether you and I are born again Christians on the basis of this observable love toward all Christians.
Later there is a final apologetic on Christ's high priestly prayer, "that they may be one, just as you, father are in me and I in you, that they may also be in us, so that the world may believe that you sent me."

he notes:
Now comes the sobering part. Jesus goes on in this 21st verse to say something that always causes me to cringe. If as Christians we do not cringe, it seems to me we are not very sensitive or very honest, because Jesus here gives us the final apologetic. What is the final apologetic? "That all of them may be one, Father, just as you are in me and I am in you. May they also be in us so that the world may believe that you have sent me." This is the final apologetic.

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 Jesus is stating something else which is much more cutting, much more profound: We cannot expect the world to believe that 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.