Atheism does not resemble the central members of the “religion” cluster, so if it wasn’t for the fact that atheism is a religion by definition, you might go around thinking that atheism wasn’t a religion. That’s why you’ve got to crush all opposition by pointing out that “Atheism is a religion” is true by definition, because it isn’t true any other way. Which is to say: People insist that “X, by definition, is a Y!” on those occasions when they’re trying to sneak in a connotation of Y that isn’t directly in the definition, and X doesn’t look all that much like other members of the Y cluster.
Arguing by definition is usually a way to sneak in a connotation.
