This episode is a gem for one primary reason, and that is the performance given by Ellen Burstyn, who played Sister Sarah, once Heath's love interest and now a nun. Of all his flames, she is the most charismatic, steadfast, and sincere.
Of course, the writers had a big hand in this as well, because her character's dialogue was some of the deepest we've seen in the show. When she tells Heath, sitting around a solitaire campfire, "there's no gap to fill" in her life since her decision, it marked one of the most genuine love scenes I've ever seen in a Western.
Karen Black's contribution was deliciously creepy, but again, less so in my opinion than Barbara Stanwyck's, who for her part, wasn't trying to be.