If they ran on a platform of, say, fundamentalist religious ethics (whatever their interpretation of that may be) and were elected based on that, then they certainly have the right to vote as they see fit based on those views. They don't have the right to violate the first amendment, but that doesn't prevent their votes on bills from being influenced by their own personal religious beliefs.

In general, I'm less concerned with the idea that there may be a few religious nutjobs that get into Congress than I am with the problem of those types of beliefs being disproportionately over-represented due to voter suppression laws and gerrymandering in (mostly) red states.

What I want is for people to be able to vote for representatives who represent common sense communal/geographical districts, and the number of representatives should accurately reflect population breakdowns, rather than being inherently handicapped in favor of rural areas. If that were the situation, I would expect to still see a handful of religious zealots make it to Washington, but I feel that with accurate representation it would be fewer than you currently see, and they'd probably be less pandering to that one niche.