If you’re a mindless tribalist, the “tribe” it’s based on is the thing that maters the most, and for some of those their tribe or at least and important part of its identity is a specific religion (often it’s even a specific sect within a major religion).

If you actually use your brain for thinking (instead of just as a cranium filling mass that keep it from collapsing into a vacuum) then it is indeed irrelevant the religion The Handmaid’s Tale is based on.