Why can't an unsaved person believe?
The unsaved cannot believe because they are in spiritual darkness; the god of this world has blinded them. Being after the flesh (old Adam), they mind the things of the flesh and have no concept of the Spirit. This is the condition of the lost individual.
Les explains that when a person is still out in the world, they are in spiritual darkness. They are blinded by the god of this world so that they cannot believe. Because they are after the flesh (the old Adam), they mind the things of the flesh. Naturally, their concerns are only the things of this world: they have fleshly appetites, fleshly vision, materialism, and no concept of the Spirit. In contrast, those who live after the Spirit mind the things of the Spirit. This is the condition of the lost individual, and it stands to reason they cannot believe.