About how the gospel is known only to believers…

And even if our gospel is veiled, it is veiled to those who are perishing. † The god of this age has blinded the minds of unbelievers, so that they cannot see the light of the gospel of the glory of Christ, who is the image of God. † For this reason they could not believe, because, as Isaiah says elsewhere: † "He has blinded their eyes and deadened their hearts, so they can neither see with their eyes, nor understand with their hearts, nor turn--and I would heal them." † Isaiah said this because he saw Jesus' glory and spoke about him. † For we do not preach ourselves, but Jesus Christ as Lord, and ourselves as your servants for Jesus' sake. † Whoever loves his brother lives in the light, and there is nothing in him to make him stumble. † But whoever hates his brother is in the darkness and walks around in the darkness; he does not know where he is going, because the darkness has blinded him. † I write to you, dear children, because your sins have been forgiven on account of his name. † 

Verse References In The Order Presented Above -- 2 Corinthians 4:3 ; 2 Corinthians 4:4 ; John 12:39 ; John 12:40 ; John 12:41 ; 2 Corinthians 4:5 ; 1 John 2:10 ; 1 John 2:11 ; 1 John 2:12 (NIV)