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" – that is because God is light and Jesus Christ makes that light visible to us.  It was physically true at the transfiguration (Matthew 17:2).

Light is also a metaphor for God's nature.  He enables things and people to be seen truly, without any shadows, without illusion (James 1:17).  Not only does God see everything, but His presence illuminates the true nature of everything so that every sin is seen as black and righteousness is seen as white.  So if we claim to be in a relationship with God but are wilfully living in sin, we simply prove that we are living a lie; we have no fellowship.  Our claim to fellowship with God may impress others or satisfy ourselves, but it is a delusion.

'Walking in the light' means that we are living in a way which pleases our Holy God.  Then, we have a true fellowship with Him and a true fellowship with other children of God.  But, as we all know, we are not totally sinless, so does that put us back into the darkness?  The answer is that as we allow His light to shine on our lives, we can see the sin, confess the shame of our sin, and the blood of Jesus Christ (which atoned for all our sins when we first came to Him) will continue to atone for every sin as we repent (1 John 1:9).  Otherwise it would be impossible for sinful human beings to relate with God the Father.

One of the most precious truths of the gospel, is that the blood of Jesus Christ makes us pure from everything that is dirty, dark, unclean and unrighteous.  The original wording affirms that His shed blood is continually active in purifying the repenting believer, assuring us of fellowship with Him.  Indeed, admitting that we have sinned is evidence that we know that God's light is shining upon us, stirring our conscience and igniting our desire to be clean again.  So, do not cower in the shadows when you can confess and know that the blood of Jesus keeps on atoning for your sin and leaving you pure in God’s sight.