Psalm 14.1 offers a true and accurate description of atheists: "The fool has said in his heart, there is no God."
Christians seem to love to quote the above verse from Psalms. Sometimes, I think this verse is popular because it allows them to call atheists "fools" and imagine that they can avoid taking responsibility for doing so - after all, they are just quoting the Bible so it's not really them saying it, right? Even worse is the part they don't quote - but not because they don't agree with it. They often do, but I don't think they want to be caught saying it directly because that's harder to defend.
Do Atheists Say There is No God?
Before getting into how this verse is used to insult atheists, we should first make note of the fact that the verse doesn't really do what Christians want it to do: it doesn't technically describe all atheists, nor does it necessarily describe only atheists. First, this verse is narrower than most Christians realize because it doesn't describe all atheists. Some atheists merely reject believing in gods, not necessarily the possible existence of any gods - including the Christian god. Atheism isn't the denial of any and all gods, just the absence of belief in gods.
At the same time, the verse is also broader than Christians seem to realize because it describes any and all theists who reject this particular god in favor of other deities. Hindus, for example, don't believe in the Christian god and, despite being theists, would qualify as "fools" according to this biblical verse. Christians who use this verse in order to attack or insult atheists are thus grossly misinterpreting it, which only serves to support the idea that they are using it for the purpose of being insulting rather than as some neutral, objective description of atheists.
You Are Responsible For What You Say
It's been my experience that Christians choose to pick out this particular verse (and just the first portion of this verse, too) in order to get a free pass at insulting atheists without having to be held accountable for their insults. The idea seems to be that since they are quoting the Bible, the words ultimately come from God and thus it's really God who is being insulting - Christians are simply quoting God and thus can't be criticized in terms of ethics, civility, tolerance, etc. This is a poor excuse, however, and fails to justify what they are doing.
These Christians may be quoting another source for their words, but they are choosing to deliver those words and this makes them responsible for what they say or write. This point is made stronger by the fact that no one takes everything in the Bible in the exact same literal manner - they pick and choose, deciding how to best interpret and implement what they read based upon their beliefs, prejudices, and cultural context. Christians cannot eschew personal responsibility for their words simply by saying that they are quoting someone else, even if it is the Bible. Repeating a charge or allegation doesn't mean that one isn't responsible for saying it - especially when it is repeated in a manner that makes it look like one agrees with it.
Do Christians Want Dialog, Or to Express Superiority?
Calling someone a fool simply because they don't agree about the existence of a God is no way to initiate a conversation with a stranger; it is, however, a great way to communicate the fact that one isn't interested in real dialogue and only wrote in order to feel better about oneself through attacking others. This can be demonstrated most dramatically by asking if the writer agrees with the second part of the verse, which declares that "They are corrupt, they do abominable deeds, there is none that does good." Although few Christians who quote the first portion of the verse rarely go so far as to include the second sentence, no atheist should ever fail to keep in mind that it is always there, hanging unspoken but nevertheless assumed, in the background.
If the Christian doesn't agree with the second part of the verse, then they admit that it's possible to not agree with something in the Bible. If that's the case, then they can't claim that they are required to agree with the first part - but if they do agree with it, then they must admit that they can be held responsible for saying it and can be expected to defend it. If they do agree with that second part of the verse, on the other hand, then they should can be expected to defend that and demonstrate that none of the atheists they are talking about "does good." They can't get out of this by saying that it's in the Bible and therefore must be accepted as true.
Christians who cite this verse are implicitly stating that atheists are corrupt, do abominable things, and don't do any good in the world. This is a pretty serious accusation and not one which can or should be allowed to pass by unchallenged. Despite numerous attempts, no theist has ever conclusively demonstrated that belief in their god is required for morality - and in fact there are many good reasons to think that such a claim is simply false.
It's easy to call someone a "fool" for not accepting your beliefs, but it's much harder to demonstrate that their rejection is mistaken and/or ill-founded. That may be why some Christians focus so much on the former and not at all on the latter. They prattle on about how it's "foolish" not to see that there must be "something more" out there, but don't look to them for anything like an argument about how or why we should see this. They can't even read and interpret their own religious scripture reasonably, so how can they be expected to read nature reasonably?