Let us hope that none of us is this "blind." From my experience as a minister, it is possible that Christians can be what David describes as a "fool." One may say, "Well, since my conversion, I have never said that there is no God." Maybe we have and never realized it!
How could that happen? "Fool" here is nabal. Remember the story of David and Abigail? Abigail's husband was named Nabal, and he was a fool. It means someone who is contemptible, someone who is empty. It does not mean "an atheist" or one who has no contact with God. It does not even mean that such a person does not see God in His creation. The fool that David describes here may readily admit that God is Creator and claim that this belief plays a major role in his life.
This person, this "fool," though not an atheist, lives as if he believes no God exists, either to bless with reward or to curse with punishment. A nabal is not stupid; he is not a person who does not reason at all. He is a person who reasons wrongly. A nabal is a person who chooses or assumes to ignore the fact of God's authority over his life. He sees God as an "absentee landlord" who may be safely disregarded because he assumes that God is not really active in His Creation. Now that is foolishness!
In biblical contexts, foolishness can be sin! The fool's problem is not with his brain but his heart. The fool is capable of grasping the things of God, but he possesses no real fear or reverence for God and the things of God. This results in nothing less than a "practical atheism." Even though he may readily admit that God is Creator, he lives his life as though God is nowhere around. He has produced a dichotomy between what he intellectually knows and the way he lives. God says such a person is a fool. He is, in reality, saying in his heart, "There is no God."
That is sobering because any of us can fall into this state, as Psalm 14:5 implies: "There [fools] are in great fear, for God is with the generation of the righteous." Like us, the fool is aware of God. When the punishment, the curse, for sin comes—when God begins to reveal Himself as the Judge of sinners—then the fool, because of what He knows of God, also knows great fear. If he truly thought, "There is no God," the fear would not exist, but he knows that there really is a God, though his life belies it.