There are reasons the man who denies God’s existence is a fool (Psalm 14). First, he is foolish because he cannot know his position is the true one. He can believe it, but that will not make it rational or true. Second, his belief leads him to live as if there is no one to call him to account. If he’s wrong, such a course is ruinous at best. Third, his course eventually leads him to mistreat those the Lord feels for most acutely, those among whom God dwells. Unbelief is not just a denial of God, but an assault on Him.
Verse 5 is difficult. Who are those “overwhelmed with dread”? There is no good answer. The text seems to say that they are the wicked because they know God is present among those who have taken refuge in Him. But if they don’t believe in God, why would this bother them? Perhaps the Psalmist is voicing his doubt that anyone truly disbelieves in God, but only act as if they do. Another scholar suggests the verse is mistranslated, that it should read ‘the wicked have formed a club, but God is not among them.’
Another way to look at this Psalm is to see it as a description of the unbelieving believer. When we act like an unbeliever, we deny by our actions the existence of God. As Gerald Wilson observes: “When we fail to acknowledge the suffering of the rest of our world, when we seek to preserve our own ‘favored status’ at the expense of those who are less powerful than we are, when we simply try not to think too hard about how our abundance is related to the poverty of others, then the judging words of Psalm 14 are directed to us” who by our lives have foolishly presumed there’s no one who notices, and no one who will hold us accountable.