You are the epistle of Christ through which His salvation can spread through the whole world!
Paul is saying that many people hear the Gospel but still are not enlightened by that. These people had made their choice to continue living in sin, therefore, satan, the god of this world had blinded their minds so that the light of Gospel remains unveiled to them, regardless of how hard somebody tries to share the gospel with them. Nevertheless, Paul and his partners presented to others the brightest light of the Gospel, – that is Christ Himself, who is the image of God. Paul was convinced that people should be able to see God in Christ, so he preached Christ and instead of preaching about himself.