One of the earliest signs that a person doesn't really understand what atheism is comes when they spell "atheism" or "atheist" with a capital A in the middle of a sentence. In English, this is only grammatical with proper nouns and thus this signals that the person imagines atheism to a be a proper noun — in other words, some sort of ideology or religion like Christianity or Objectivism. When you see someone inappropriately capitalizing atheism, beware.
Little Things Are Important
At first blush it might appear petty to worry about grammar, but it's not at all petty in this case. It's one thing to make minor mistakes — everyone does and a certain amount of tolerance of mistakes should be maintained. Consistently spelling atheism and atheist with a capital A in the middle of the sentence is not, however, a minor spelling issue.
This matters because it matters if a person falsely believes atheism is an ideology rather than simply the absence of belief in gods. This not only means that they don't even comprehend the basic definition of atheism, but are in fact working from a definition which will cause them to draw all sorts of incorrect conclusions about atheists. Most of the myths about atheism do, in fact, stem from thinking that atheism is a belief system.
So if you see a person capitalizing atheism and atheist in the middle of a sentence, you need to cut short the conversation and educate them about what atheism really is. You need to do this before the conversation starts winding down blind alleys that lead nowhere — a common occurrence with Christians trying to critique an idea about "atheism" which has no connection to reality.
A Sign of Respect?
The most creative excuse I've seen for misspelling atheism and atheist is that it's supposed to be a sign of "respect." I've been assured that the person really did understand that atheism is merely an absence of belief in gods, but was convinced that atheism deserved to be treated with the same respect as Christianity and thus should be capitalized just as Christianity is capitalized.
This excuse is so weak that I hardly know where to begin with it. Perhaps it's sufficient to point out that capitalization in English has nothing whatsoever to do with "respect" and everything to do with separating out proper nouns. If a person really does believe that capitalization is done out of "respect," then they don't even comprehend basic English grammar and you should beware of them even more than if they merely didn’t understand atheism.
If someone wants to "respect" atheism, they should simply make the effort to comprehend what it is and is not before presuming to make declarations about atheism or atheists. It's not that hard.