The fool says in his heart, “There is no God.”
Psalm 14:1a esv
The fool does not necessarily declare his atheism publicly with his mouth (or with a pen), but rather takes this position in his heart. In fact, he may not have explicitly denied the existence of God at all, but he does “live and behave as though God takes no notice of human behavior” says Craig Evans.* “In short,” Evans continues, “the declaration ‘There is no God’ is an expression of practical atheism, of living as though God is distant and indifferent.” I agree that practical atheism is in view here. The Hebrew term for fool refers to a moral orientation and not a philosophical position. It is the folly of a non-God-fearing life that the psalmist here bewails.
Although the past decade has seen a dramatic rise in explicit and public expressions of atheism — including advertisements on big city buses — practical atheism has been around since the beginning. Craig Evans, in the article cited below* goes on to note in his conclusion that
“the practical atheism of the biblical period and late antiquity is much like the atheism of today. Most humanity believes that God exists, but much of humanity lives as though God does not exist or, if he does, he takes little interest in human affairs. The atheism of today is much more than what we see in writings and pronouncements of atheist celebrities, such as Sam Harris and Richard Dawkins. …The fools are not limited to those who openly state, “There is no God”; they include those who profess belief in God but live as though he does not exist.”
*Craig A. Evans: The Fool Says In His Heart, “There Is No God” – Atheism In The Bible And In Late Antiquity, in The City, Publication of Houston Baptist University, Winter 2015, pp.13-23.