Why does a person's heart become hardened?
A person's heart becomes hardened through repeated wrong choices: he makes a choice, lives with it, then decides wrong again, and in the process becomes hardened. Man hardens himself by exercising free will, choosing to put God out of his thinking, though this occurs under God's sovereign control.
Les explains that the hardening is not something God does by frying the heart; man does it to himself. The pattern is repeated wrong choices: a person makes a choice, lives with it, and when the next situation comes, he decides wrong again and becomes hardened. God puts him on the spot, and although Scripture says God did it, the person hardens his own heart. Les applies this to mankind as a whole: when people choose to put God out of their thinking and think only in the human realm—secular humanism—their foolish heart is darkened. This happens under God's sovereign volition, yet the exercise of free will remains ours.