A characteristic feature of the so-called "new atheism" is that it opposes itself not simply to Christianity, or even to theism, but to religion. One of its central contentions -- meant to deflate particularistic claims to uniqueness -- is that religion admits of a unified theory: though Christianity differs from Islam, say, both -- and indeed all tokens of the type -- are explicable in terms of the same basic mechanisms. Yet, this understanding of religion as a transcultural and transhistorical universal is a distinctively modern, Western one. This paper seeks to locate the emergence of the contemporary concept of “religion” partly in European philosophical and theological debates over the threat to Christianity perceived to be posed by secularization. Developing a line of thought advanced by Tomoko Masuzawa, I argue that the invention of “religion” (as a genus) and the “world religions” (as its species) in the nineteenth and twentieth centuries, like the creation of deism in the seventeenth and eighteenth, served apologetic goals but also exposed the resulting formations to new forms of criticism. Whereas atheism is often said to be “negative,” in that it can be defined only by reference to what it rejects (and thus does not constitute a unitary viewpoint), I argue that “atheism” and “religion” are dialectically co-constituted categories, and that lines of influence continue to push in both directions. The distinction between “religion” and its other(s), I conclude, is better understood as emerging out of, rather than as the underlying cause of, ongoing debates about atheism.