Soon after becoming a follower of Christ, I ran across Psalm 14:1 (and Psalm 53:1). The author writes: “The fool says in his heart there is no God.” Having just transitioned from non-theism, I thought this verse somewhat harsh. Afterall, I wasn’t a fool at 30 and now suddenly wise at 31. All I could do at the time was put these words in the cognitive-dissonance category and move on. Today, I wholeheartedly agree with the writer. I realize I had years ago misconstrued a fool with one who is slow of mind. I similarly had the wrong idea about what it means to be wise. So what are wisdom and foolishness and why is the latter a defining attribute for the man who denies God?

Let’s set some terms at the outset. Wisdom is poorly defined these days to mean not much more than common sense or good judgment. But what is common is not always good, and what is good, not always common. Such definitions are subjective and unclear. Taking an objective and scriptural position: Wisdom is the right application of knowledge such that it aligns with the Lord’s intentions. The closer the alignment, the wiser we are. When we are foolish, we fail to apply rationality in ways that align with His purpose. The fool’s thoughts are askew from God’s.
Foolishness may arise from intelligence and careful deliberation. Wisdom may be discerned quickly and without forethought. Neither qualities are necessarily dependent upon our natural talents. A person might be highly intelligent and foolish, or have a sub-100 IQ and be wise. Since believers and unbelievers alike cover the full spectrum of intellectual ability, mental acuity is not a prerequisite, nor the lack thereof a preclusion, for acknowledging God. Something else drives the alignment of our understanding, regardless of how limited or how vast that understanding might be.
Scripture is clear that everyone ought to know God. Psalm 19:1 says: “The heavens declare the glory of God, and the sky above proclaims his handiwork.” Romans 1:18-20 builds on this: “For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth. For what can be known about God is plain to them because God has shown it to them. For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse.” Not only is the denier of God without excuse, given the sufficient evidence for His existence revealed in creation, but Paul says unrighteousness leads to a suppression of truth. One commentator clarified this suppression as “Truth held in the bondage of immorality.
From Augustine to Nietzsche, noted throughout history is the primacy of will over reason. When desire fixates on what is contrary to knowledge, a capitulation of the intellect takes place. We twist the truth (rationalize) or enter into denial and stubbornness. Our understanding is at the mercy of our will and affections. This conflict takes place at our noetic core; a place Scripture refers to as the heart.
We might think the heart as that part of the soul merely concerned with passion and desire. But Jesus said that “out of men’s hearts, come evil thoughts” (Mark 7:21). Luke 1:51 refers to the proud “thoughts of their hearts.” So the heart is also involved in the higher parts of the brain. The heart can soften creating a humble desire for the light of truth. It can also harden where the intellect is wrestled into a darkened position and pinned to the mat. Ephesians 4 describes how obdurance leads to this futility of mind: “Now this I say and testify in the Lord, that you must no longer walk as the Gentiles do, in the futility of their minds. They are darkened in their understanding, alienated from the life of God because of the ignorance that is in them, due to their hardness of heart.” The intellect is downstream from the heart.
Another word oft misunderstood, unrighteousness, put simply, is the state of being in sin, a state contrary to the divine law which follows necessarily from God’s nature. Unrighteous acts are sinful acts. Righteousness, on the other hand, is freedom from sin. Paul states in Romans 3: “No one is righteous, no not one.” But it is interesting to note what directly follows: “No one seeks God.” Again we see the linkage from unrighteousness to a foolishness that denies the Creator.
Piecing things together we can show a complete causal chain: 
Unrighteousness -> the heart hardens, grows dishonest, desiring darkness and things contrary to God -> the will wrestles the intellect into ignorance of God and misalignment with His intentions -> foolishness
Righteousness -> the heart softens, grows honest, desiring the light of truth -> the will stimulates the intellect into a diligent and open inquiry of God and His aims -> wisdom
The Holy Spirit plays a critical role in the causal chain to wisdom. Abiding in Christ allows us to walk by the Spirit who counsels us on precisely how to align our thinking with God’s intentions. This counseling process, where His spirit testifies with our spirit, involves our noetic core in ways beyond what I’m prepared (or able) to write on here. Nevertheless, this interaction is an essential element of the process, and I would be remiss not to mention it. 
There is also a feedback loop to consider: Foolishness leads to more sin and unrighteousness. This loop has the potential to drive our spiritual state into utter depravity. Similarly, there is a feedback loop with wisdom within the causal chain. John 3 illustrates this concept: “The light has come into the world, and people loved the darkness rather than the light because their works were evil. For everyone who does wicked things hates the light and does not come to the light, lest his works should be exposed. But whoever does what is true comes to the light, so that it may be clearly seen that his works have been carried out in God.” Doing what is in alignment with God’s aims draws us further into the light.
We see the writer’s claim is justified. The foolish deny God, and their state of ignorance is not unexpected given Scripture. Righteousness leads to wisdom, unrighteousness to foolishness. The Holy Spirit plays a crucial role in leading us into the light. Sin is dangerous as it gives birth to foolishness and more sin, creating a positive feedback loop driving one into darkness. God has carefully ordained the world with enough knowledge of himself so we are without excuse. As Pascal wrote: “There is enough light for those who only desire to see and enough obscurity for those who have a contrary disposition.” But there are greater forces at work than mere evidence upon our intellect pushing us off the fence of general revelation.