John started his letter saying that God is life, eternal life, and that a living fellowship with God is only possible when we receive His eternal life. Now John says that God is light. He is the light-giver (Genesis 1:3) but He is more than that, He is light. As Hebrews 1:3 says, "The Son is the radiance of God's glory", so God is light and Jesus Christ makes that light visible to us. It was physically true at the transfiguration (Matthew 17:2).