The word of life, first God is light ; 1 John

1 John 1:1-10
That which was from the beginning, which we have heard, which we have seen with our eyes, which we have looked at and our hands have touched—this we proclaim concerning the Word of life.
We proclaim to you what we have seen and heard, so that you also may have fellowship with us. And our fellowship is with the Father and with his Son, Jesus Christ. We write this to make our joy complete.
God is light; in him there is no darkness at all. If we claim to have fellowship with him and yet walk in the darkness, we lie and do not live out the truth.
If we claim to be without sin, we deceive ourselves and the truth is not in us.  If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness.

John's theme is 'the Word of life'. At the beginning of his Gospel John spoke of the person who is God's Word of life: Jesus Christ, someone John actually knew, Here too he speaks of his own experience: this living embodiment(實施例) of God's Word could be seen and touched. The word is both a person and a message: the two are inseparable. John had seen God's message of eternal life in Chris, had heard it from him, and now he joyfully declares it with a confidence born of certainty.
What is this message? First, that 'God is light'. 
Why?
Jesus Christ is the Lord, is the word of life. John and other apostles have seen, touched and heard him.
God is light there is not darkness, if anyone work in the darkness he is lie and do not live out the truth, same as claim to be without sin.
This is the light that generates life and dispels darkness. Anyone who claims 'fellowship' or communion with God yet lives 'in the dark' is telling a lie. But to say we never sin is equally false. If we live in the light we will recognize and confess out sins. God can be relied on to keep his promise of forgiveness and cleansing.
How?
Be joyful for God, Jesus Chris is the light and we may be forgiven from our sin when we confessed our sin.