Technically, John is still speaking of obedience as evidence of a relationship with God through Christ. The test of love here is not a whole new category of evidence. However, when he repeats the list of evidences in the second half of the book, he will distinguish this as its own category. So, we can go ahead and begin to speak of love as another evidence of genuine conversion.
But, this is really a continuation of the test of obedience and devotion. Love is the attitude and approach to life and relationships that the law attempts to prescribe. In that sense, the command to love one another is not new; it is the essence of the law. It is what we find when we read between the lines. Anyone who truly obeys Jesus commandments will exhibit love.
And as a quality of true believers, it is an especially helpful diagnostic. We have seen that people who really walk in the light reject sin. That can be a difficult thing to measure in one’s life. The struggle against sin—even in the most faithful of believers—is a daily reality. We have also seen that the true believer obeys. Again, the distinction between joyful followers of Christ and legalistic rule-keepers can be subtle at times.
However, love is the tangible measurement. Love is the driving force that compels us to reject sin and obey. It is our love for God that motivates that desire. And, it is the quality of love towards others that distinguishes obedience from legalism.
In his Gospel, John quotes Jesus’ positive expression of the command to love one another. Here in his epistle, John gives us the negative expression. If we do not love one another, we are fooling ourselves in our attempt to follow Christ. We are not walking in the light. We are stumbling around in the dark.