The difficulty with this claim is the simple fact the none of our beliefs are held in isolation from one another. For example, if I do deny the existence of Trolls then I am positively saying something about our sources of knowledge within the created world. Since many peasants have a folk belief in Trolls (in a variety of different cultures), then I am making the claim that a proper basis of belief is not traditional folk belief, but rather is something else. That is a positive assertion, rather than a negative one. Moving to the question of God's existence; saying that God does not exist presupposes a Materialist metaphysic (i.e., only material entities and causes exist) and therefore actually isn't a negative at all, but a positive assertion that there is nothing beyond the material world.
Most Atheist/Materialist types don't get this because of how they think about their belief system and the nature of knowledge. What they assume is that a world of purely material causes is somehow obvious and that belief in anything beyond it is irrational. Hence, they think that Theists simply randomly and irrationally assert the existence of something beyond the world when they have no justifiable way of making said assertion. It is a pure act of "faith" (as they understand the term), which means for them, irrational belief in something one cannot prove.
There are several difficulties with this view of things, not least is that the Materialist worldview is actually one possible in a post-Christian culture (the Japanese are still Animists, even though they have developed high technology). Genesis 1 empties the universe of gods and goddesses in the way that other ancient cosmologies don't. If you reject YHWH, then all you have left are material causes and laws. Hence Atheism and Materialism presupposes "faith" in a particular worldview that has historically not been "obvious" to no one outside of our post-Christian culture.
Now comes the second problem: namely Atheism/Materialism have no ability to give an epistemic account of themselves. In other words, as Alister McIntyre has pointed out, worldviews only work insofar as they are able to give an account of themselves as believable. Theists are able to give an account of themselves. Theists claim that they can know that God exists because he 1. accounts for the existence of the laws of nature and of morality (which are easy to explain with a God, but not without one). 2. That God has revealed himself to humans. Humans who know of God's existence do not randomly assert that there's something beyond the world of material causes, but rather explain their knowledge of God by stating that he has in a sense broken through the veil of material causes and made himself known. Something beyond our immediate experience is knowable because he has entered into our experience.
Therefore the difficulty of Atheism/Materialism lies in the fact that it has no way of giving an account of itself epistemically the way that Theism can. First, it asserts the laws of nature and morality hold good without a law giver. Whereas in all other cases where we can visibly discern the structure of the casual order, design and order presupposes a law-giver and designer, according to Atheists, we are to believe that when it comes to the created order as a whole, there is one a huge exception. Secondly, it cannot give an account of how it knows that there are only material causes. Whereas Theists have the mechanism of supernatural revelation that makes the unknowable knowable, Atheist have no such mechanism. For Materialists to know that there are only material entities and causes, they would, (so to speak) have to climb into heaven and find it empty. This is something that they clearly cannot do and therefore they cannot account for their lack of belief in the supernatural. Their argument is circular: Material entities and causes alone exist, because we can only know material causes and entities.
This being said, the structure of belief in Materialism and Atheism is actually identical with the fideism that they accuse Theists of. Without actually being able to give any account of the basis of their belief, Atheists and Materialists randomly assert that there is no God and that there are only material causes, when they have absolutely no ability to give an account of how they know this. For this reason it is Materialism which is actually fideism, and not Theism.