Plantinga’s proper-function account of warrant is one theory among many and has well-known counterexamples. Isn’t building an argument on a contested theory of warrant building on sand?

Proper functionalism is contested, and this whole claim is an argument to weigh, not a settled result. The core move does not depend on the details, though. On any theory of warrant that ties it to reliability or to the belief’s origin, whether religious belief is warranted will depend on whether it is true, which is the conclusion drawn here. The argument is robust across theories of warrant that make warrant depend on facts outside the believer’s head, which is most of them.