How does the Bible describe a fool?

TL;DR:

The Bible describes fools in detail, showing how their foolishness reaches to every area of their lives, and ultimately brings harm to themselves.

The Bible describes a fool as someone who denies God and lacks the fear of the Lord, which is the foundation of true knowledge. A fool is characterized by an unwillingness to learn or accept correction, caring only about expressing his own opinion. His speech is marked by slander, folly, and rashness, often leading to conflict and personal ruin. A fool's behavior is impulsive and lacks self-control, showing frustration and emotions without restraint. His heart is occupied with iniquity, and he repeatedly returns to his foolish ways. A fool engages in sexual immorality, disrespects his parents, and disregards sacred offerings to God. In the New Testament, foolishness is associated with sin and defilement, and Jesus contrasts the wise person, who follows His teachings, with the fool, who does not, leading to inevitable failure. Despite the fool's destructive path, there is hope for the fool who repents and builds a solid foundation on Jesus' words.

from the old testament

The Bible is our foundation for wisdom in every area of our lives. When we ignore the wisdom that God freely gives us (James 1:5), we walk in foolishness. The Bible says that the fear of God is the beginning of knowledge (Proverbs 1:7). From this verse we can infer that lacking fear of God is the beginning of foolishness. In fact, the Bible says, "The fool says in his heart, 'There is no God'" (Psalm 14:1). The height of foolishness is to deny God.
A fool is unwilling to learn or be corrected. He only cares about expressing his own opinion (Proverbs 18:2).
A fool utters slander (Proverbs 10:18–19), proclaims folly (Proverbs 12:23), and answers before he hears the whole question (Proverbs 18:13).
A fool's words invite a fight, and his mouth is his ruin (Proverbs 18:6–7).
A fool's frustrations are known immediately, and he vents his emotions without restraint (Proverbs 29:11).
A fool's heart is busy with iniquity (Isaiah 32:6), and he returns to his foolishness continually (Proverbs 26:11).
A foolish person commits sexual immorality, leading to his own destruction (Proverbs 6:32; 7:7–27).
A fool loves his folly (Proverbs 10:23; 15:21).
A fool does not respect his parents, despises his father's instruction (Proverbs 15:5, 20), and has no respect for sacred offerings to God (Proverbs 14:8–9).

from the new testament

The Bible is clear that foolishness is sinful, evil, and defiling, even categorizing it with adultery, deceit, theft, and pride (Mark 7:21–23).
Jesus compares the foolish person to the wise person, saying, "Everyone then who hears these words of mine and does them will be like a wise man who built his house on the rock. And the rain fell, and the floods came, and the winds blew and beat on that house, but it did not fall, because it had been founded on the rock. And everyone who hears these words of mine and does not do them will be like a foolish man who built his house on the sand. And the rain fell, and the floods came, and the winds blew and beat against that house, and it fell, and great was the fall of it" (Matthew 7:24–27).
It is important to note that the fool that has already been addressed is different than the word Jesus used when He says, "whoever says, 'You fool!' will be liable to the hell of fire" (Matthew 5:22). The word Jesus uses there is raca, which expresses condemnation. It would be similar to someone today saying, "damn you!" Jesus is not saying that we shouldn't call someone out for their foolishness. Confronting people about their sin is something Christ gives us specific instructions on how to do (Matthew 18:15–20; 1 Thessalonians 5:14). Rather, He is saying that we cannot take God's place and condemn someone to hell for his actions.

implications for today

In the end, a fool brings destruction on himself. Any foundation other than Jesus Christ will fail when storms come. While there may be momentary pleasure in foolishness, it only brings difficulty and heartache, separating you from God and from other people. We become foolish when we ignore what the Word of God says, forgetting that God sees all that we do. We become foolish when we forget God's goodness and try to walk according to our own wisdom rather than leaning on God's wisdom (Proverbs 3:5–8; 28:6–7). The Bible warns us not to trust a fool because of their love for foolishness (Proverbs 26:6). Of course, there is always hope for the fool. Anyone walking in foolishness need only repent and begin fearing the Lord, building a foundation upon the words of Jesus, and he will rest in wisdom upon solid ground.

understand

The fool denies God's existence and lacks the fear of God, which is the foundation of knowledge.
A fool refuses to learn or accept correction, focusing only on expressing his own opinions.
A fool's impulsive and sinful actions lead to personal ruin and repeated folly.

reflect

Are there areas in your life where you're acting as if there is no God? How can you develop a deeper reverence for God?
When have you resisted learning or correction? What steps can you take to be more open to advice and growth?
How have impulsive actions or words negatively affected you? How can you cultivate greater self-control?

engage

What signs of foolishness do you see in society? How can we help each other avoid these behaviors?
Why is accepting correction difficult? How can we create a supportive environment for growth?
What does building your life on Jesus' teachings look like? How can we apply this practically?