I know but that's what crossed my mind at first. Can offer a better one now. "I doubt he's a good christian" or "He isn't a good christian, is he?" think it's closer to the idea behind the french thing (ie)"Il a pas l'air très catholique" as it questions eventually more a moral attitude, a set of values based on catholic christianity rather than the appearance, "doesn't look like a catholic" a more word by word translation. Does it suits you better?