You don't know how many times I've heard Christians use this verse from Psalm 53:1 as a way of "demonstrating" that I'm somehow a fool. It allows theists to say "My Bible says you're a fool for not believing in god so that proves you're a fool. Ha ha." I always tell them that if I were to make up my own religion I too would include admonishments that preempted any desire to disbelieve.
Many believing Christians put so much emphasis on the written word of the Bible, especially the inerrantists. To them, the Bible truly is the word of god and it therefore must be true in its entirety. So if the Bible says the fool denies god, then it must be true, because the Bible said so. What wonderful logic they employ. Even if a theist believes the New Testament probabilistically demonstrates Jesus to have risen from the grave, it still doesn't entail that every book of the Bible, and every verse in it is therefore true. That's the mistake fundamentalists make that liberal Christians don't. Every verse in the Bible has to be assessed on a case by case basis and if it is not backed up by archaeological evidence and extra-biblical sources, it should not be considered historical.
I tell Christians that using a verse like Psalm 53:1 on me is as futile as me using a verse from the Qur'an would be on them. Neither of us accept the authority of those "holy" books and so the brute force of their verses fall flat. And sometimes I counter argue Psalm 53 with biblical verses that condone slavery and sexism, which there are plenty, and there is always a denial that those verses mean what they say. It's always the case, it seems to some Christians, that the Bible never means what it says when the verses are condoning harmful things like slavery and sexism. It always means something else, something more politically correct and in tune with our modern sensibilities.
Psalm 53 is just another masochistic view from down below contemplating how corrupt and vile and unworthy of life mankind is. What some Jew wrote thousands of years ago in his desert dwelling no more represents the truth about the atheist than the ignorant modern day fundie who conflates atheism with communism. The real fool is the one who accepts the "truth" of the Bible without a critical examination of it, because upon critical examination, the biblical narrative is completely at odds with modern science and centuries of archaeological findings. Anyone who affirms its inerrancy must deny science, archaeology or suffer from some massive headache caused by cognitive dissonance.
Or be a total hypocrite.
I propose an alternative to Psalm 53:1 for skeptics: The fool says in his heart, the Bible is inerrant.