Non-believers hear this charge frequently from Christians, and it carries about as much credibility as "I know you are...but what am I?"
I can understand why this is such an attractive retort from the faithful to the atheist.
* It uses its own book to validate itself, making broad claims about salvation and judgment (which is much like quoting from Harry Potter books to warn humankind against the sinister snares of Voldemort).
* It uses provocative language ("fool").
* It allows the user to assume a position of moral superiority.
* And it allows the messenger to feel powerful as he wields a not-so-veiled threat of punishment for non-compliance.
To Christians standing on the words of Psalm 14, I must remind you that Islam is just as vehement in its assertion that all who reject Allah are fools and destined for destruction ("And whoever desires other than Islam as religion - never will it be accepted from him, and he, in the Hereafter, will be among the losers." - Qur'an 3:85).
Now...If an Islamist accosted you with the charge that you were a fool for not believing in Allah, how much gravity would you give the argument? I suspect you'd care very little, as you are already an atheist in regards to Allah. In fact, you're an atheist in regards to all of the thousands upon thousands of deities that humankind has worshiped throughout history, the only exception being the Christian god that almost undoubtedly was taught to you as fact by your family and culture.
What's truly foolish? To build one's position on an ancient document penned by anonymous authors in a deeply illiterate part of the Middle East? Or to accept that the information in that document lacks the historical, scientific and moral authority to declare that anyone is a fool? Isn't Psalm 14:1 the equivalent of having the National Academy of Sciences publicly called foolish by the producers of "Battlefield Earth?"
For my part, I think these particular protests are so vehement because believers are secretly panicked over the weakness of their own position. They seek confirmation. Agreement. Validation. Their protests aren't about God. Their protests are about them.
After all, an omnipotent God would be unfazed by the criticisms of mere human skeptics. And God certainly wouldn't require backup by social media cowboys in love with the CAPS button. In fact, a truly all-powerful, just and benevolent deity could either dispatch all dissenters with a whisper or provide immediate and undeniable proof that all biblical claims are true. After all...he's God.
Alas, we're instead given silence from the shepherd and left with only the ringing condemnations from the flock. And as they attempt to frighten and shame us with the scriptural label of "fool," we reject the Book of Psalms as a credible platform for judgment, and we lament the legitimate foolishness that attempts to build a claim so tall upon a foundation so weak.
"Tell me there is a God in the serene heavens that will damn his children for the expression of an honest belief! More men have died in their sins, judged by your orthodox creeds, than there are leaves in all the forests in the wide world ten thousand times over. Tell me these men are in Hell; that these men are in torment; that these children are in eternal pain, and that they are to be punished forever and forever! I denounce this doctrine as the most infamous of lies." - Robert G. Ingersoll