What does choosing secular humanism do to a person's heart?
Choosing secular humanism—putting God out of one's thinking and thinking only in the human realm—darkens a person's foolish heart. It is self-hardening: man does it to himself, yet under God's sovereign volition, as a response to the exercise of free will.
Les explains that when people choose to put God out of their thinking and think only in the human realm—what he calls secular humanism—their foolish heart becomes darkened. This is not God frying the heart; man hardens himself by his own choices. As with the example given, each wrong decision leads to further hardening, yet it operates under God's sovereign volition. The darkened heart is a response of God to man's exercise of free will, for all mankind's actions under God are based on that free will. Though God is sovereign and in total control, the exercise of the will remains ours.