Films are made about politics, crime, war, love, abuse, sex and the rest of life, itself.
Why NOT religion? If, for some reson, this were not allowed, then we'd never have seen films like The Ten Commandments, Ben Hur, The Nun's Story, Song of Bernadette and many others. If religion were a taboo subject, we'd never see how humanity has been affected by religion. And, after all, it has been religion that has molded much of the world's culture. Example, why is Western Culture so different from Eastern Culture? For the most part, comparative religion is most of the answer.
Besides, WHO, exactly draws that boundary line? Where, exactly would that boundary line be drawn? Could the film and television series, M.A.S.H. have been done without Father Mulcahey? Could the film about Boy's Town have been done without Father Flannagen? Would The Shoes of the Fisheman have to be rewritten with the protagonist cast as a, what?... a rock star?