When Rufus Wainwright was asked to write a closing theme song for “Rebel Hearts,” a documentary about a renegade order of socially activist nuns that opens this weekend, he didn’t require the preamble that virtually any other singer-songwriter would have. It was intergenerationally personal for him, as he was already intimately familiar with the subject matter, thanks to his grandfather’s girlfriend having been part of the order of nuns that got in trouble with the Catholic church in the 1960s, and having heard her story over the years.

The result is “Secret Sister,” a compelling song that evokes both spiritual mysteries and calls to concrete action, and which appears on the “Rebel Hearts” soundtrack along with another original song, Sharon Van Etten’s opening “Conjunction.” Variety has the premiere of an excerpt from Wainwright’s song (below), along with some of the historic and modern footage and animation from director Pedros Kos’ film,...