“By this all people will know that you are my disciples, if you have love for one another.” (John 13:35 ESV)
The distinguishing mark of genuine Christians is love for their brothers and sisters in Christ. Those who experience Christ’s love by faith in His gospel are to show that Christian love to one another.
Christians are urged to display this chief characteristic: “Love one another with brotherly affection. Outdo one another in showing honor” (Romans 12:10). “Above all, keep loving one another earnestly” (1 Peter 4:8). This is how we know we are saved: “We know that we have passed out of death into life, because we love the brothers” (1 John 3:14).
But Jesus says it is not only Christians who will know they are His disciples when they love each other. He tells us that by loving one another “all people will know that you are my disciples.” Christ-like love witnesses to the world that we are followers of Jesus Christ. When the world observes our love for each other, they should be able to distinguish in some sense that we are Christians.
The late Francis Schaeffer says it is as if “Jesus turns to the world and says ‘I’ve something to say to you. On the basis of my authority, I give you a right: You may judge whether or not an individual is a Christian on the basis of the love he shows to all Christians.’ In other words, if people come up to us and cast in our teeth the judgment that we are not Christians because we have not shown love toward other Christians, we must understand that they are only exercising a prerogative that Jesus gave them.”
So we must take this command to love one another very seriously. For when we obey and love others as Christ loved us, we are showing the world what being a Christian is all about. And Jesus has given them the right to watch and pass judgment on our love for one another.