Light and Darkness, Sin and Forgiveness
5 This is the message we have heard from him and declare to you: God is light; in him there is no darkness at all. 6 If we claim to have fellowship with him and yet walk in the darkness, we lie and do not live out the truth. 7 But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus, his Son, purifies us from all sin.
One of the big themes in the Gospel and letters of John is light and darkness. John says that God is light and in him there is no darkness at all. Another big theme in John is fellowship, which comes from the. Greek word "koinonia". Fellowship is a word that means a deep friendship and closeness that comes about through one's relationship with God. When John says "God is light" he means that he is holy and pure. Light is also a metaphor for the truth. When something is brought into the light it is exposed for what it really is.
On the other hand "darkness" represents sin and evil. Those who walk in sin, walk in darkness. The deeds they do are not usually in broad daylight but in the dark. They remain secret and hidden, much like the opposite of truth which is lies. So John is confronting a well known heresy in that day called "Gnosticism". This false belief was a form of dualism that said the body was bad and the Spirit is good. There were two forms that this manifested. One, since the body was bad, it was to be beaten into submission. Desires were seen as bad and this was where the "ascetics" gravitated to. The other side of the coin was that since the body was bad and the spirit was good, one might as well indulge in the bodily desires because the body would die anyways.
So John is saying, if you say you have fellowship with God, who is the light, and then walk in darkness, you are living a lie. This is a relationship between the fellowship with have with God who is light, and the way we live our lives. Jesus said, "I am light of the world", so as we have fellowship with Jesus, this also brings us into all truth.
What can we learn from this? Light and darkness have nothing in common. Just as God has nothing in common with sin. We have fellowship with God and participate in His nature through the power of the Holy Spirit. So when we engage in sin and seek out darkness, we purposely move away from the fellowship with Jesus. And this affects our relationship with other believers. Light has nothing to do with darkness. This doesn't mean we are sinless, but we learn to distinguish the light from the darkness and truth from error. When we walk in the light we walk in freedom and joy.