Why can't an unsaved person believe the gospel?
An unsaved person cannot believe the gospel because he is in spiritual darkness and blinded by the God of this world. Being after the flesh, or old Adam, he minds fleshly things and has no concept of the Spirit, which explains his natural inability to believe.
Les explains that the lost individual is set in contrast with the saved individual. The unsaved person is still out in the world, in spiritual darkness, blinded by the God of this world so that he can't believe. Since he is after the flesh—the old Adamic nature—he naturally minds the things of the flesh: fleshly appetites, materialism, and no concept of the Spirit. The saved person, by contrast, lives after the Spirit and minds the things of the Spirit. This is why the unsaved cannot believe the gospel; their condition is one of spiritual blindness and fleshly orientation.
In whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, who is the image of God, should shine unto them.