"The one who loves his brother abides in the light and there is no cause for stumbling in him.  But the one who 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."

Clear lines are drawn, in John's eyes, as to the difference between the light and the darkness; the difference between the friend of God and the enemy.

Jesus told us that people would know we belong to Him because of our love.  Here, John tells us that I can discern in myself where my heart is, and what the temperature of my spirit is simply by judging what my love covers.

Do I know how close I am to God?  Seems very hard to tell.  But John informs us that all I have to do is figure out whether or not I love the ones that surround me. 

I can tell you how much I love my neighbor.  And that tells me how much God is changing me, or whether or not I abide in the Light.

If you seek righteousness - learn to love.