Atheists sometimes point out that there is no reason to believe in god exactly because such a belief is a matter of faith. As faith is defined as "belief without evidence", this is equivalent to saying that you should not believe in god because there is no evidence for his existence. This is obviously based on the presumption that you should not believe in the existence of something if there is no evidence for it.
The concept of faith comes in handy in a few cases in which I personally feel that it's bloody obvious that a certain view is bonkers, but also know that my making the statement "that's obviously bonkers" will not be awfully convincing to you if you do not agree anyway. Take "natural rights". It should be self-evident that they don't exist. Yet there's a famous document, revered by many, which states that it's self-evident people are endowed with "certain inalienable Rights". What if you want to argue that such rights don't exist? You can point out that there is no evidence for their existence. They're a matter of faith.
Closely related, the philosophical position called "moral realism" (which holds that some moral propositions are objectively true, independent of psyches that recognize them as true) is a matter of faith. So is the belief that objects can be said to have an objective quality (in the sense of "higher" or "lower" quality), implying that you can say that movie X is objectively better that movie Y and that if you don't agree you are mistaken. These positions should not be assumed to be true, as there is no evidence for them. You should not say that you're absolutely certain they're wrong either, as evidence for them may come along in the future. I have no idea what evidence for natural rights or moral realism or objective quality would even look like, but that doesn't mean it's impossible it will come along.
I should add that this post was in part inspired by reading Brad Taylor's post "Natural Rights Don't Exist" (via), which doesn't really deliver on the title, but succeeds in showing that the nonagression principle can lead to very undesirable consequences. Yet once you're arguing in terms of consequences, you've already left the question whether natural rights exist. He tries to weld the two issues together, but does not, I thnk, succeed. Perhaps he should simply have said that there is no evidence for the existence of such rights.