Why can't lost people believe the gospel?
Lost people cannot believe the gospel because the God of this world, Satan, has blinded their minds. The gospel is hidden to them, and the attractive, glimmering things of this world blind them so the light of the glorious gospel of Christ cannot shine unto them. They do not even realize they are lost.
Les explains that the gospel is hid to them that are lost, and they have no idea they are lost, like a sheep until it is too late. In verse 4, the crucial dilemma is that the God of this world has blinded the minds of them who believe not. Les connects this to Ephesians 2, where they are steeped in deadness and the works of the flesh. Satan has them so totally blinded. Les illustrates that a bright light can blind, just as night driving lights blind more with age. The bright, glimmering, attractive things of this world blind lost people, so the light of the glorious gospel of Christ, who is the image of God, is there for them, but they cannot see it.
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.