Foolishness is primarily a moral condition rather than a state of intellectual ability. There are some very bright fools out there!
Scripture declares in no uncertain terms that foolishness starts with denial. The fool is the person who says to themselves, the person who lives their life as if there is no God.
Wisdom starts with fearing God, and to know God results in insight (Proverbs 9:10). Therefore to declare that there is no God sentences the declarant to a state of folly since they cannot even start to have wisdom or understanding.
More than this, such a person’s denial of God and refusal to assign to God His rightful place in their lives and the world has devastating results. It leads to a corruption of their personhood and a downward spiral of their deeds.
This downward spiral is on display in Romans 1:18-32 where the truth about God is exchanged for a lie and so such people ‘claiming to be wise, they became fools…’ (vs22)
Wrong believing leads to wrong living, and right believing leads to right living.
May you and I always fear God and keep God in His rightful place in our lives. That’s the way to wisdom, may we never depart from it and may we continually pray that God would in His grace reveal Himself those who are currently denying the truth about God.