The Mormon thing hurts him with protestant fundamentalist wackos who are a small, yet irritatingly vocal, part of the GOP base. But once he has the nomination, who are they going to vote for, Hillary? They'll hold their nose and vote for Mitt, because the alternative is far worse. The general election is about going after moderates, and they couldn't care less about whether he's a Mormon or not. The impression that he's distancing himself from his church might even be a plus in their eyes.
Glad you have it all figured out.