God is light; in him there is no darkness at all

1 John 1:5-10
God is light; in him there is no darkness at all.
Walk in the light, we have fellowship with one another, and the blood of Jesus, his Son, purifies us from all sin.
If we claim to be without sin, we deceive ourselves and the truth is not in us.
If we confess ours sins,  he is faithful and just and will forgive us out sins and purify us from all unrighteousness.
What is the message? First, that 'God is light'. 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 ever sin is equally false. If we live in the light we will recognize and confess ou sins. God can be relied on to keep his promise of forgiveness and cleansing.
Why?
Because everyone is sinner, original sin when we are born. so confess our sin will be forgiven, if we claim to be without sin, it is deceive ourselves.
1 John was written to counter some early form of 'Gnosticism' being propounded by people who were once church members, but had now withdrawn from the group. They linked to think of themselves as intellectuals - processing a superior knowledge of God. Contrary to the traditional Jewish and Christian holistic view of the world, they drew a line between the spiritual (good) ad the material(evil). In practice this often led to immorality, because nothing the body did could tarnish the purity of the spirit. It also led to a denial of Christ's human nature, which was either 'make-believe' or only temporary. They reasoned that Christ, being spirit, could not have died.
How?
Confess my sin, Jesus Christ may save us and forgives us sin, then we may fellowship with God and others in light.