In the third season finale of Downton Abbey, which airs Sunday night on PBS, series creator Julian Fellowes puts the cap on perhaps the most uneven—but definitely the most spiritual—season yet of his colossal hit. He's more than made good on his (somewhat unofficial) declaration that faith would enter the show's multiple storylines.

On the surface, there is a great deal more discussion of mere religion this year. It's driven at first by the presence of Tom Branson, the Crawleys' ex-chauffeur and new son-in-law, who is both Irish and Catholic. After being exiled for revolutionary activities in his homeland, he informs his new, culturally Anglican family that his soon-to-be born child will be christened Catholic. Meanwhile, the poor ...