Alvin Plantinga, while setting up his evolutionary argument against naturalism in Where the Conflict Really Lies, writes (page 311):
Now it is not clear that naturalism, as it stands, is a religion; there is enough vagueness around the edges of the concept of religion for it to be unclear whether naturalism does or doesn't belong there. But naturalism does serve one of the main functions of a religion: it offers a master narrative, it answers deep and important human questions. Immanuel Kant identified three great human questions: Is there such a person as God? Do we human beings have significant freedom? And can we human beings expect life after death? Naturalism gives answers to these questions: there is no God, there is no immortality, and the case for genuine freedom is at best dicey.
I am puzzled by Plantinga's reservation to call naturalism a religion. What is this vagueness he refers to?
It is easy for me to accept a definition of a religious expression as any statement attempting to answer any of those three questions Kant posed or similar ones. That is, anyone attempting to answer any of them would be representing a religious position. The only way not to represent a religious position would be to not talk about those questions at all.
Now, if I limit myself to Western atheism, I can see the development of it perhaps originating in the 18th century that has at least parallels with the development of the forms of Protestant Christianity. Hence I suspect the same social influences in opposition to Catholic Christianity that motivated Protestantism motivated Western atheism.
However, I wonder if I am missing something. I am taking this further than Plantinga does, although Plantinga does ambiguously call naturalism, the form of extreme atheism he is concerned with, as a "quasi-religion" (page 311). I don't see why he doesn't go further except that it might distract from the presentation of his argument against naturalism.
Hence the question in the title: Is Western atheism a kind of Protestant religion?
Plantinga, A. (2011). Where the conflict really lies: Science, religion, and naturalism. OUP USA.