Is it ever OK to question a political candidate's religious beliefs?

Of course it is. If Jim Jones (of the ill-fated People's Temple) were alive and running for office, could we challenge him on his religious beliefs? What about Sun Myung Moon? He considers himself to be a Christian and is very politically conservative. David Koresh? Marshall Applewhite? A Satanist? A Wiccan?

The only question is "Just how bizarre are Mitt Romney's religious beliefs?"
As Jacob Weisberg, editor of the influential (and liberal) online journal Slate, wrote:

Someone who refuses to consider voting for a woman as president is rightly deemed a sexist. Someone who'd never vote for a black person is a racist. But are you a religious bigot if you wouldn't cast a ballot for a believing Mormon?
     ...Objecting to someone because of his religious beliefs is not the same thing as prejudice based on religious heritage, race, or gender.
Weisberg reasons, correctly, that while all people are free to run for elected office regardless of religion: "it is not bigotry to say that some religious views are deal-breakers."
     . . ."I wouldn't vote for someone who truly believed in the founding whoppers of Mormonism... [which] is based on such transparent and recent fraud."
It is not, Weisberg argued, that Romney "is a Mormon," but the he "has never publicly indicated any distance from Church doctrine."