The difference is that atheists far less frequently than theists have a belief system that centers on the belief in the non-existence of god.
Theists frequently have their religious beliefs at the center of their system of beliefs about the reality of our universe (and beyond). Atheists typically do not. I think that is the important distinction here. "Atheism" doesn't purport to describe anything about the nature or purpose of our existence, though it can color some beliefs on those topics. It merely precludes certain possibilities, or at least calls them into question.
Some atheists, of course, do make the cause of atheism their animating principle in life. But usually when religious folks bring this up, it's in an attempt to try to say that "atheism" is a religion and therefore atheists are hypocrites for criticizing or looking down on theists. Which is dishonest, to say the least.
There cannot be that many true athiest who do not. There are probably fewer theist who do. Being a theist does not prove there is a God. Or that would be the only proof and evidence needed. But even the usage of the word athiest is the basis of a personal belief system. If we accept that all humans are athiest, until they are not, at what point do we not have a personal belief system? If we have evidence and experience that changes us from athiest to non-athiest is that the point we have a personal belief system? What if we do know and then loose faith in our belief system, what was the knowledge we had? That would be starting out not having one, then having one, and then loosing it. Then the question would be did we ever have one to begin with which is the default claim being made. What if we start out with knowledge and then it is changed by experience or the evidence being taught to us by others, and we loose such knowledge altogether?
Shouldn't it be the opposite? If you have evidence, you do not need faith.
If existence is more than material and physical, then no. One person cannot contain all knowledge. That is why faith is needed. If there is no spiritual side to existence would we even need faith and to an extent mysticism in our vocabulary? Faith is the evidence we do not have, but we do not need all the knowledge to exist either. Faith and belief would not exist in our minds if they were not needed. We also extend belief and faith in the knowledge that others have as a mechanism to create a smooth and working society.
The argument has been: can we replace all the unknown with knowledge. If you accept that, are you deceiving yourself and discriminating on which past claims you are willing to accept? Who is making the judgment call, when one stops believing or even flat out makes the claim that God does not exist, when we know we do not have all the knowledge available in the universe? Can we say that one had never believed? If the question is not applicable, because we are allegedly a blank state at birth with nothing, and then we start receiving knowledge, I do not accept that. I am not sure we can specify at what time we can or cannot have knowledge. The point has always been what do we do with knowledge. Because we are capable of thinking on our own we but heads with others and even our own experiences. We form our opinions and make judgment calls that we may or may not have to live up to. What we get from others, information wise, either forms us or it does not, but we cannot complain, even if we disagree with the knowledge presented to us. Even if humans are totally corrupt and are considered evil, we cannot eradicate that knowledge in totality. Religion does not make knowledge any less wrong or right. Religion is just another form of culture we pass down from generation to generation via flawed humans with a limited knowledge base.
The point about morality and ethics is because we have the knowledge of good and evil. If we did not have that knowledge morality and ethics would not exist either. If the universe has to have a set of laws why would that rule out humans not also be bound by laws that could not change. If we could not change what we do, would that not make having the knowledge of good and evil pointless? We cannot be forced to do both at the same time. We could be forced to know and experience either all the time. If everything was always good there would be no need for change and by extension evolution itself. If laws help correct the evil, but only evil happened what are laws for? If we had knowledge of good and evil and that knowledge was unnecessary, why have it? If the universe itself did not need good or evil to function what point would humans have for knowing good and evil? The only reason I can see is that it gave humans the power to choose their own outcome in their existence in the universe, even if it seemed unnecessary in the totality of the universe. The universe lives and dies on unchangeable laws. Is not the knowledge of good and evil an important deviation from the rest of the universe? If the universe was not bound by laws and just chaotic and by extension evil, would that not invalidate science and place it in the same category as ethics and philosophy?