I think you are confusing two different issues here. Atheism vs Theism is about Belief, Gnosticism vs Agnosticism is about Knowledge. My wife and I are both agnostic atheists. Neither one of use Know whether or not there is a god, but neither of us belive that one exists based on the available evidence and rational marshaled as justification for his/her/its existence. It is possible to be a Gnostic Atheist (Knowing and Believing in the absence of a deity), as well as an Agnostic Theist (believing in god without actually knowing). From my perspective, the truly scary to me are the Gnostic Theists who claim to know for certain that god exists, not because of any empirical evidence, but simply because... Their counterparts, the Gnostic Atheists at least have a view that is consistent with observable phenomenon and are generally willing to be convinced of their error with sufficient evidence. I've had Gnostic Theists on the other hand tell me flat out that there is no evidence they would accept of god's nonexistence to even open up the possibility that they might be wrong. That kind of absolutism is truly dangerous.
Atheism is not a religion, it is the absence of religion and therefore a "true believer" in atheism is an oxymoron. It's like if you ask someone what there favorite cola is. The majority will say Coke, a close second will be Pepsi, some percentage will name far less popular colas, and some will say they don't like cola at all. That last group is the functional equivalent of an atheist. To say that their favorite cola is "None" is not really correct because it presumes that they like cola at all, which is not the case.
That being said, there are assholes in any group, and one should not confuse the views and actions of the asshole as representative or indicative of the group. And in defense of some atheists I've seen accused of being militant (my wife being one), what believers often perceive as being militant is actually being unapologetic. My wife's family has on several occasions attempted to engage my wife in religious discussions only to get frustrated when she turns there attempts at conversion (which no matter what they claim, was the true purpose of these conversations) into a dialog where she explains her beliefs and tries to make them understand her view. They view her attempts to turn the tables as being disrespectful and rude because they start from the assumption that god exists and any discussion of the possibility that he might not be real is inherently wrong and disrespectful to god. As the previous poster pointed out, there are lots of things people believing for which there is no credible evidence. Just because someone believes in something does NOT mean I have to show respect for that belief. However, lack of respect for the belief does not grant me permission to show disrespect to the believer. The religious in this world enjoy a privileged status in most society and many view that privilege as their right, instead of as an artifact of previous intolerance of different religions or the non-religious. Therefore they have a hard time not seeing my lack of respect for their belief as a lack of respect for them as a person.