Summary: We may either love one another or hate one another; there is no halfway in this situation. God calls us to love. Failure to love is hatred.
“Beloved, I am writing you no new commandment, but an old commandment that you had from the beginning. The old commandment is the word that you have heard. At the same time, it is a new commandment that I am writing to you, which is true in him and in you, because the darkness is passing away and the true light is already shining. Whoever says he is in the light and hates his brother is still in darkness. Whoever loves his brother abides in the light, and in him there is no cause for stumbling. But whoever hates his brother is in the darkness and walks in the darkness, and does not know where he is going, because the darkness has blinded his eyes.” 
Retro is in. It appears that if we live long enough, that which was once discarded as out of date will come back into fashion. That is true in the world, as fashions that were jettisoned become popular again, as music that was identified with a particular group becomes popular again or as food that fell out of favour with the most comes back into favour. What is true in the world is equally true among the churches. Fads spring up with amazing regularity, and heresies once roundly defeated again plague the people of God. One fad that seems to spring up with dismaying regularity is reducing the Body of Christ, the congregation of the faithful, to an organisation. Thus, many professed people of God play musical church, moving from one congregation to another as though God had nothing to say about the matter. Such an act is tacit admission of a failure to love, arising from a failure to know God.
The text is focused on knowing God. In particular, those who claim to know God are called to “walk in the same way in which He walked.” And the walk of faith is not mysterious! That walk is, however, exciting; it reflects the character of Him whom we call “Father!” John is called the Apostle of Love with reason. He emphasises the necessity of love as the essential mark of the follower of the Christ. Tragically, this defining characteristic is absent in the life of much of modern Christendom. In fact, we would be hard pressed to demonstrate love as a defining characteristic among the professed people of God in this day when church has been reduced to an optional feature of religious life. Before we examine John’s commands, we must define what we are looking for.
I have the distinct impression that in the mind of most contemporary church members, love is defined primarily—if not exclusively—as an emotion. Perhaps we Christians are not able to define adequately what we mean when we speak of “love,” but whatever definition we seize upon tends toward what we used to call a “warm, fuzzy.” We argued that love made us feel good; love was revealed in feeling good. Whether we have thought through the implications or not, in modern parlance love is often a condition controlled by others. Others have the ability to show us love, but love does not lie within our purview when considering others.
That such exclusivity reflects an aberrant view becomes evident through even a cursory review of Jesus’ teaching concerning love. Love for the Son reveals relationship to the Father. Jesus challenged the religious leaders when they avowed that God was their Father, “If God were your Father, you would love me, for I came from God and I am here. I came not of my own accord, but he sent me” [JOHN 8:42].
Love leads one to be sacrificial in life. Recall Jesus’ teaching on this matter. “I am the Good Shepherd. I know my own and my own know me, just as the Father knows me and I know the Father; and I lay down my life for the sheep. And I have other sheep that are not of this fold. I must bring them also, and they will listen to my voice. So there will be one flock, one shepherd. For this reason the Father loves me, because I lay down my life that I may take it up again. No one takes it from me, but I lay it down of my own accord. I have authority to lay it down, and I have authority to take it up again. This charge I have received from my Father” [JOHN 10:14-18].
Love is revealed through obedience. Listen to Jesus on this matter. “‘If you love me, you will keep my commandments. And I will ask the Father, and he will give you another Helper, to be with you forever, even the Spirit of truth, whom the world cannot receive, because it neither sees him nor knows him. You know him, for he dwells with you and will be in you.