"Beloved, I am not writing a new commandment to you but an old commandment which you have had from the beginning; the old commandment is the word which you have heard.  On the other hand, I am writing a new commandment to you, which is true in Him and in you, because the darkness is passing away, and the true light is already shining.  The one who says he is in the light and yet hates his brother is in the darkness until now."

John is painstakingly describing the revolution.

Is it a new idea to walk in the Light of Christ?  No.  In fact it is a very old concept for the people John is writing to.  They are familiar with the idea that God is light.  When Moses walked away from the tabernacle of God, his face shined.  They know the concept of walking in the Light and being changed because of it.

But it is new to them at the same time, because they never expected to be changed because of the Light.  They have always expected the priest to be changed, never the common man.

So the brother is changed by the Light.  The father, the mother, the son, the friend, the co-worker, the priest, the leper...  The darkness is passing away, and behold, Light...

If you seek righteousness, seek the Light.