Atheism is not a philosophy; it is not even a view of the world; it is simply an admission of the obvious. In fact, "atheism" is a term that should not even exist. No one needs to identify himself as a "non-astrologer" or a "non-alchemist." We do not have words for people who doubt that Elvis is still alive or that aliens have traversed the galaxy only to molest ranchers and cattle. Atheism is nothing more than the noises reasonable people make in the presence of unjustified religious beliefs.An "atheist," of course, is then a person, presumably reasonable, making (skeptical) noises in the presence of unjustified religious beliefs, making it a rather poor term for identifying something--or really much of anything--about that person. It's a term with little utility, and what use it has is not only unfortunate, it is also dangerous.
The reason is that atheism is a term of negation, as Harris makes clear by analogies. Atheism negates theism, a belief in God or gods. By extension, then, it is tempting to call those people who believe in God or gods theists, which is accurate and semantically appropriate. Asymmetric by definition, unlike "atheist," an argument that "theist" should not be a word doesn't hold water; I just go out of my way to avoid using that term, at times taking pains to do so.
Now, to disclaim, I am not a proponent of taboos being placed on words or ideas, and I'm hardly going to stand and moralize about the use of this term or that in one context or another. Words have meanings, denotative and connotative, and in the societies in which those meanings exist, there are complex social rules for using or not using various terms. Likewise, there are artful, and more commonly inartful, ways for violating these norms, replete with appropriate and inappropriate outrage as the cases may be. That disclaimed, I'm getting to the thrust: Don't say the "Th" word: theist.
Before explaining, I should note that by extension, I would recommend against using the "Ath" word (atheist) as well except that I can't. Because of the current status of thorough cultural domination of those who accept some form of theistic belief, at least in many places including the United States (from which I write) the "Ath" word is altogether too useful as a concise and totally unfortunate label. Instead, I advise avoiding taking the "Ath" word to heart and attempting to make it an identifiable part of one's identity. Doing so is a dangerous invitation to use the "Th" word in exactly the way that motivates me here.
Now, to explain, consider that the "Th" word doesn't describe anybody, even if it is an accurate label for what may be a significant majority of the population of the world. Even if every Christian, every Hindu, every Muslim, inter alia, is technically a theist, none of them identify themselves that way. When someone, almost always an atheist who has embraced the term as part of her identity and is engaging in strident (online) activism, uses the "Th" word, we're left sorely wondering who exactly she is talking to. My guess is that they're talking to other ornery atheists who count "their atheism" as part of their identities and thus talking about those labeled with the "Th" word.
Particularly, to build upon that, Christians identify as Christians; Hindus identify as Hindus; Muslims identify as Muslims, etc., and it is likely that only a small percentage of people in any of those categories are familiar with the "Th" word. I certainly never knew of it until I started paying attention to atheist literature. Perhaps I'm mistaken, but it seems plain that if your intended audience has to look up the word that you are using to identify them, then you are not actually trying to reach them.
That should leave us wondering exactly why so many e-defenders of "atheism" are so quick to use the "Th" word, but haven't you guessed it? It others the people who are not atheists. The "Th" word, then, which has some limited academic usefulness, is socially being used to mean what might be captured by the term "anatheist," meaning a "not-atheist." A quick search of the use of the "Th" word on Twitter will hopefully reveal beyond doubt exactly what I mean here.
Notice how this depends upon the error of accepting atheism as a part of one's identity, ignoring Harris's oft-quoted comment that "atheism is not a philosophy; it is not even a view of the world." It also depends upon a tendency to engage in exactly the kind of parochial, tribal behavior that the "Th"-word-using atheists are rebuking. As is often the case, this hypocrisy doesn't necessarily take away from the validity of their arguments, but it certainly distracts from it.
There are many ways to rationalize, and even to justify, the use of the "Th" word. One arises from the infrastructure of Twitter, where this behavior is common. On Twitter, there is a forced premium on almost obscene brevity, and the "Th" word describes a general category of people with only six characters (seven, if pluralized). Another defense is that it is a broad-stroke term that catches everyone that believes in God or gods under a single umbrella. This is useful for talking about them, then, but not necessarily to or for them. Another common justification strikes closer to home: it is the logical negative counterpart to the "Ath" word. Whatever the justification, though, the question isn't whether or not it is valid so much as if it is worth the ease with which it turns into an othering term and thus whether or not othering believers serves achieving positive societal goals.
So far as I can tell, it is easy to engage in othering, but it is far less often productive than destructive to do so. No one wants to be othered, and groups that realize they are being othered often get a sense of entrenchment and unity not otherwise available. To wit, note the (mostly online, largely on Twitter) growing "atheist community" (this should be plural, since there are already the seeds of denominationalism running rampant there, its own problem not to be tackled here). Simply put, there are better ways to communicate. Tackle specifics: particular religions or sects, particular arguments from theologians or apologists, particular beliefs or traditions. Save the broad brushes for places that they make sense, such as topics like faith--"pretending to know what one doesn't know," to follow Peter Boghossian's brilliant observation.
Thus, I hope you'll join me. Don't say the "Th" word.