Below is an excerpt from Francis Schaeffer's 1971 book, The Mark of the Christian. A link to the complete online text of the book follows.

... It is a universal mark that is to last through all the ages of the church till Jesus comes back.

What is this mark?

At the close of his ministry, Jesus looks forward to his death on the cross, the open tomb and the ascension. Knowing that he is about to leave, Jesus prepares his disciples for what is to come. It is here that he makes clear what will be the distinguishing mark of the Christian:

My children, I will be with you only a little longer. You will look for me, and just as I told the Jews, so I tell you now: Where I am going, you cannot come. A new command I give you: Love one another. As I have loved you, so you must love one another. By this all men will know that you are my disciples, if you love one another." (John 13:33-35)

This passage reveals the mark that Jesus gives to label a Christian not just in one era or in one locality but at all times and all places until Jesus returns.

Notice that what he says here is not a description of a fact. It is a command which includes a condition: "A new command I give you: Love one another. As I have loved you, so you must love one another. By this all men will know that you are my disciples, if you love one another." An if is involved. If you obey, you will wear the badge Christ gave. But since this is a command, it can be violated.

The point is that it is possible to be a Christian without showing the mark, but if we expect non-Christians to know that we are Christians, we must show the mark.

 

http://www.ccel.us/schaeffer.html