The indefinite article is used to show that there is something else available. For example, we might say, “I am a male,” as opposed to “female.” We could also say “I am male,” dropping the indefinite article. Both are correct, but one is categorical, and the other is descriptive.
I also subscribe to a vegan lifestyle. When I describe myself I could either say, “I’m a vegan,” or drop the article altogether and say, “I’m vegan.” Both are perfectly acceptable, per our grammar rules. But one denotes something that the other doesn’t. And we can apply this to atheism as well.
By saying, “I’m an atheist,” we are categorizing ourselves, saying that there are other groups out there, but we are a part of this group. We belong to the atheist group. The indefinite article creates an “us vs. them” idea. There is no real atheist group or community. We atheists are alike only in one very finite aspect of our lives. Many of us feel no special connection to other atheists solely because they are atheists.
If we say, on the other hand, “I’m atheist,” and omit the article, then it is merely a description. “Hi, I’m Rayan. I’m vegan. I’m bearded. I’m white. And I’m atheist.” This sounds better to me because it only provides a description of me.
This is not to say that people won’t imagine categories when we say, “I’m atheist.” They probably will. And some may even say in their heads, “Oh, he’s an atheist, one of them!” as if we really were a group. That just comes with the territory. Many people will judge us the same based on uttering either statement until such a time that religion truly doesn’t matter. But for now, implying that we are part of this group or that group instead of merely describing ourselves might be perpetuating the negative stereotypes against people who have no belief in god.
It should be noted, on the other hand, that this idea just now popped into my head. I created a quick opinion on the matter. It’s subject to change. And to be fair to everyone, I might not even know what I’m talking about on this one because I might not have thought this all the way through. Feel free to ridicule this idea, offer alternatives, support it, or anything else you may feel the urge to do.