"And this is the message we have heard from Him and announce to you, that God is light and in Him there is no darkness at all.  If we say that we have fellowship with Him and yet walk in darkness, we lie and do not practice the truth; but if we walk in the light as He Himself is in the light, we have fellowship with one another, and the blood of Jesus His Son cleanses us from all sin."

There is a different standard for the follower of Christ:  walk in the light.

What does it mean to walk in the light?  It means that we walk in the same place that Jesus walked, or in the same manner that Jesus walked. 

John is describing the journey toward Christ-likeness, toward righteousness.  No longer do you bear any resemblance to your old self.  Now you look more and more like Christ.  Now you are effected more and more by the light.  It is the same in the natural world:  If you stay in the sunshine long enough, you will get a sun burn.  You are changed by the light.

The fellowship that John is writing about is basically a well-tanned fellowship.  It is not a collection of people that have arrived at perfection, but one that is in the active process of becoming different because of the Light.

If you desire righteousness - walk how Jesus walks.