In John 13-17 Jesus shares one last evening with his disciples before he goes to the cross. One last time to prepare them for the time when he would not physically be present with them. A time to reemphasize the most important teachings, making sure they are fresh in their minds.
In John 13:34-35 Jesus says: “A new command I give you: Love one another. As I have loved you, so you must love one another. By this everyone will know that you are my disciples, if you love one another.” This was a new commandment, something Jesus had been demonstrating throughout his ministry, but now sets down as something they are to do as well: "Love one another".
Jesus had earlier identified the greatest commandment from the Old Testament Law as loving God with all ones heart, soul, mind and strength. This command is followed by loving ones neighbors as oneself. This second command seems very similar to the new command Jesus issues here, but is actually quite different.
Under the Old Testament law I am to love others in the same way I love myself. This essentially says that I should be treating other people the way I want to be treated, similar to the 'Golden Rule'. And it for sure it is generally a good guideline for interacting with people around me.
But under this new commandment that Jesus gives, I am to love other believers in the same way that Jesus loved me. This narrows the scope to fellow believers rather than all the world. And it also changes how I am to love them. Rather than loving them like I love myself, I am to love them like Jesus loved me.
Jesus gave his life for me. If I am to love in the same way then I need to give up my life for others within the body of Christ. While I need to treat all people with respect and dignity, I need to go beyond that with my brothers and sisters in Christ and actively seek to do what is best for them within the body.
How will the world know that those in the fellowship I belong to are disciples of Christ? By the way we treat each other. If they come into gatherings and see little difference from what they would in any other group of people, then we have fallen short. But if they come in and see that we love each other with a self sacrificing love, then they should take notice and realize that there is something different in us; that we have been with Jesus.