Religious people occasionally accuse me of adhering to a faith based belief system, since being an atheist who accepts well-supported scientific theories seems like a leap of faith to them. I disagree of course. I have no faith with which to support a faith based belief system. I am still able to accept scientific explanations, but I do so on the basis of trust, not on faith. Faith requires no evidence and is considered strongest in the absence of proof. Trust is always conditional, always provisional, and is strengthened by evidence. In fact, trust requires evidence. So I am comfortable saying I have no faith, none at all. That is not a religious mindset.
Atheism is no more a belief system than "not believing in fairies" is a belief system. It is nothing more than the absence of a particular belief, that is to say a belief in any deity. What it isn't is the belief that there are no gods. This isn't a case of being picky about words. The distinction between those two definitions is important. One is a statement of faith that a negative can be proven -- which is silly. The other is merely the logical condition resulting from a lack of evidence and would apply to any other proposition as well. There is no substitution of belief built into that, not automatically anyway. Though I will concede that many atheists in Western societies do tend to have some common views on things, some of which may require atheism as a prerequisite to hold.
Sometimes I'm told that atheists are like religious zealots since both are absolutely certain and therefore agnosticism is a more reasonable belief. The trouble with that is that agnosticism isn't a belief. Agnosticism addresses a different point as it is about what is known or can be known -- not what is believed. An agnostic contends that the question of God's existence is unknown and perhaps unknowable. Therefore agnosticism and atheism are not mutually exclusive positions. For that matter neither are agnosticism and theism.
I and most atheists I've met aren't atheists because we know with certainty that there are no gods, but rather we contend that there is insufficient evidence to support such a belief. We are strictly speaking agnostic about what we know about deities. But in terms of belief, we don't have it and thus are atheists. This is in no way an unusual application of logic and people use reason this way all the time. For example, it is possible that a gamma ray burst will fry our planet tomorrow. There is no reason to think that it will happen tomorrow and therefore even those aware of the possibility go on about their lives as though it won't happen even though they don't know for certain.
Another source of disagreement can be deciding who has the burden of proof for their claim, the theist or the atheist. I would argue that the burden of proof is where it has always been, upon those making the claim. Given that atheism is the absence of a claim, it is up to theists and believers to provide evidence that supports their claim that their deity exists. Since there is no evidence to support that claim, it is reasonable to not believe, making the default position disbelief to be won over to belief as the evidence accumulates.
It is only after evidence is processed that acceptance is adopted -- which is, of course, subject to further change in order to incorporate even more evidence. As I don't have a competing belief system to supplant first, I would accept the existence of a deity or any other currently unsubstantiated claim if there was verifiable, scientific evidence. My mind isn't closed to the idea of adopting new views, but I want those views to be as factual as possible. So I utilize critical thinking and scientific inquiry to separate facts from fanciful ideas.