But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus Christ, his Son, cleanses us from all sin.
as he is in the light: It is the aim of the worshipper to become like the god he worships. God is light and always lives in the sphere of light. If we are to have fellowship with Him, we must live our lives in His sphere of light; or in this context, we must order our lives in a sphere of purity of life. We must be holy as He is holy, pure as He is pure.