‘If we say that we have fellowship with him and walk in the darkness, we lie, and do not the truth.'
Here he makes that position crystal clear. Because God is light no darkness can survive in His presence. To say that we have fellowship with Him, that we share a life in common with Him, that we live in His presence, that we have an open relationship with Him, while walking in darkness, that is, while not allowing Him to reveal to us the awfulness of our sin, and to illuminate our hearts concerning Jesus Christ, is to lie. It means that we are not acting in accordance with the truth, that our lives are a lie and that we are deceived, that we have failed to come to the light of God.
There is, says John, no way of having dealings with God until we have, as far as we are aware, brought the whole of our lives into His light. For God is not available to those who cling to sin, and live deliberately in darkness. They have no part in Him. God is light, and until we recognise that fact and respond to it by letting Him deal with us about our sin, we have no part in God. We are rejecting the truth and deceiving ourselves. It is true that to begin with our awareness of that light may be dim, and that we will grow as we come to know Him better, but as we do become more aware of Him, and do come to know Him better, that light will shine ever more brightly in our hearts and we will become more and more aware of our own sinfulness, and of the light of the Good News of the glory of Christ Who is the image of God (2 Corinthians 4:4) bringing continuing salvation to our souls. This is what being a Christian is all about.
And he could have added, Did not Jesus Christ Himself say, ‘I am the light of the world, he who follows me will not walk in darkness but will have the light of life' (John 8:12). So He too is light as God is light, and we cannot follow Him and walk in darkness.