Who is it exactly that God calls a fool? Is it only unbelievers?
What is a Fool?
One of the best known verses about a fool in the Bible is from Psalm 14:1 which says “The fool says in his heart, ‘There is no God.’ They are corrupt, they do abominable deeds, there is none who does good.” The Hebrew word used for “fool” in Psalm 14:1 is “nabal” and means “foolishness” or “senseless” and indeed it is foolish not to believe that God exists. I am not calling a non-believer a fool; God is. In the New Testament, the Greek word used for “fool” is “aphrōn” which means “without reason,” “without intelligence” or “senseless” as we read in Luke 12:20 where Jesus says “Fool! This night your soul is required of you, and the things you have prepared, whose will they be?'” In both of these cases, it is foolish to deny the existence of God and for the one who Jesus spoke about in Luke 12, it was foolish  for the rich man to deny God and at the end of life, “the one who lays up treasure for himself and is not rich toward God” (Luke 12:21).
Who is a Fool?
We have already read that the fool denies the existence of God (Psalm 14:1) because all we need to do is to look at creation and see that a creation demands a Creator, just as a painting demands a painter. Unbelievers are without any excuse in denying God’s existence because “The heavens declare the glory of God, and the sky above proclaims his handiwork” (Psalm 19:1) and “Day to day pours out speech, and night to night reveals knowledge” (Psalm 19:2) and so unbelievers cover their eyes and close their ears to the obvious fact that God must exist. The creation is what is called “general revelation” or the fact that God has revealed Himself in His creation. It would be like an ant denying the existence of the earth upon which he walks and gathers food. We really know in our hearts that God does exist; it’s just that people deny God’s existence because they either love their sin and don’t want to stop or they are so full of pride that they refuse to humble themselves and acknowledge that there is a supreme and sovereign God that rules over all. It could even be both.
Attributes of a Fool
We all make foolish decisions but that doesn’t make us a fool. There is a distinct difference between being a fool and making foolish choices. For one thing, “A wise man’s heart inclines him to the right, but a fool’s heart to the left” and “Even when the fool walks on the road, he lacks sense, and he says to everyone that he is a fool” (Eccl 10:2-3) and “Doing wrong is like a joke to a fool, but wisdom is pleasure to a man of understanding” (Prov 10:23). In the end, “a babbling fool will come to ruin” (Prov 10:8b). I admit that I have spoken without thinking and later had to apologize. I should have known that it’s better to say nothing than to remove all doubt and let people hear I was a fool since “A prudent man conceals knowledge, but the heart of fools proclaims folly” (Prov 12:23). You cannot reason with a fool because “The way of a fool is right in his own eyes” (Prov 12:15a).
Suppressing the Evidence
Contrary to what non-believers say, they have no excuse for denying the existence of God because “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” (Rom 1:19-20). Instead of acknowledging God, knowing that He exists, “they exchanged the truth about God for a lie and worshiped and served the creature rather than the Creator” (Rom 1:25). This is why the psalmist and other authors of the Bible repeat this so often that “The fool says in his heart, ‘There is no God’” (Psalm 53:1).
Avoiding the Foolish
The simple ones or the foolish ones don’t make good company for the believer which is why Solomon recommends that we “Leave the presence of a fool, for there you do not meet words of knowledge” (Prov 14:7) and so we must “Walk with the wise and become wise, for a companion of fools suffers harm” (Prov 13:20). The Apostle Paul  reminds us to “not be deceived: “Bad company ruins good morals” (1 Cor 15:33) and so we should “Make no friendship with a man given to anger, nor go with a wrathful man lest you learn his ways and entangle yourself in a snare” (Prov 22:24-25) because that man or woman is a fool. We can be led into mischief, problems, and sin by a fool if we fool his or her way but “The heart of the wise inclines to the right, but the heart of the fool to the left” (Eccl 10:2).
The Bible describes anyone who refuses to believe in God as a fool because they know in their heart that He does exist and that He will bring judgment upon everyone who refuses to humble themselves and confess their sins, repent of them, and turn to Christ and put their trust in Him. Everyone will acknowledge the existence of God someday, but for many, it will be too late, and it will be at the judgment seat of Christ (Rev 20:12-15). Sooner or later it will be at “the name that is above every name so at the name of Jesus every knee should bow, in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord, to the glory of God the Father” (Phil 2:9-11). Paul gives one last warning to everyone who refuses to bend the knee and confess that Jesus is Lord and King that it is “because of your hard and impenitent heart you are storing up wrath for yourself on the day of wrath when God’s righteous judgment will be revealed. He will render to each one according to his works” (Rom 2:5-6).
Something to read about the opposite of foolishness: The Wisdom of God 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), copyright © 2001 by Crossway, a publishing ministry of Good News Publishers. Used by permission. All rights reserved.