One such belief is materialism. Atheists often use materialism to 'to prove' there is no God. This leads to some question begging: how do we know materialism is true? This of course gets us into the realm of epistemology. Most atheists today are pretty modernistic when it comes to epistemology. Here is where a bit of postmodernism can help us. We can use there own weapons against them.
In his critique of Richard Dawkins, philosopher Alvin Plantinga states this about Dawkins' use of materialism:
But second, suppose we concede, at least for purposes of argument, that God is complex. Perhaps we think the more a being knows, the more complex it is; God, being omniscient, would then be highly complex. Perhaps so; still, why does Dawkins think it follows that God would be improbable? Given materialism and the idea that the ultimate objects in our universe are the elementary particles of physics, perhaps a being that knew a great deal would be improbable—how could those particles get arranged in such a way as to constitute a being with all that knowledge? Of course we aren't given materialism. Dawkins is arguing that theism is improbable; it would be dialectically deficient in excelsis to argue this by appealing to materialism as a premise. Of course it is unlikely that there is such a person as God if materialism is true; in fact materialism logically entails that there is no such person as God; but it would be obviously question-begging to argue that theism is improbable because materialism is true.
So why think God must be improbable? According to classical theism, God is a necessary being; it is not so much as possible that there should be no such person as God; he exists in all possible worlds. But if God is a necessary being, if he exists in all possible worlds, then the probability that he exists, of course, is 1, and the probability that he does not exist is 0. Far from its being improbable that he exists, his existence is maximally probable. So if Dawkins proposes that God's existence is improbable, he owes us an argument for the conclusion that there is no necessary being with the attributes of God—an argument that doesn't just start from the premise that materialism is true. Neither he nor anyone else has provided even a decent argument along these lines; Dawkins doesn't even seem to be aware that he needs an argument of that sort. (source)
The problem is Dawkins conceives of complex in materialistic terms and then moves on to dismiss such a concept. He assumes all is materialistic in order to prove there is no God. On less complex levels, people assume that all our knowledge must come through our senses and so if our senses cannot detect it, then it does not exist--or at least it probably does not exist.
Science by default is the investigation of natural phenomena. But a field that is limited to the investigation of natural phenomena cannot prove there are only natural phenomena. It is kind of like a fish saying “Because I cannot get out of my fish bowl and I can only investigate that which is in my fish bowl, I have proved there is nothing but my fishbowl in this world.”
Of course the issues are more complex than this and there is always more that could be said. But the assumption of materialism does not entail "proof" that there is no God because we cannot 'see' him. Of course, Dawkins would resort to parody of flying teacups and spaghetti monsters: see we cannot see them should we believe they might exist. But this is another issue for another time.