David describes this psalm as a meditation. But God inspired him to write something that God would then inspire the Apostle Paul to use in his letter to the church at Rome. (Ref. Romans 1-3). This psalm may have been written before Psalm 14; the first three verses of each psalm are the same.
Why is this emphasize in the bible? It is because at some point in time most people are fools when it comes to God. We are fools when we deny the existence of God. We are fools when we do not have time for God. We are fools when we think we are good enough for God. We are fools when we don’t seek God. We are fools when we put off dealing with our need of God. Why do we do these things? Because the hardest thing for a human being to admit is that “I am a sinner in need of a Savior”. From the age of 9 to the age of 13, I realized I was a sinner, but I didn’t want to admit it because I was afraid of what people would think of me. But at 13, I trusted Christ as my Savior and have never regretted it. My foolishness became evident when I found all the people I thought would think less of me had actually been praying that I would find Christ; they were very supportive of my decision. That’s how sometimes our minds cause us to give up on something God wants to bless us with. Of course, we believe there are exceptions for young children and the mentally handicapped; we believe God will not hold them accountable for what they are unable to understand.
Why do we believe this? Because this is what God says in His book to us, and also because God sent His one and only Son to become a human being, to live a perfect life before God, and to die on a cross as a sacrifice paying the price to redeem all humans that we may have peace with God knowing that He loves each and every one of us and to give us the promise of eternal life with Him to all who will trust in that sacrifice alone.