“The Catholic Left,” as many of us may need to be reminded, isn’t an oxymoron or the ambiguous title of a Graham Greene novel. If not well known (or even particularly existent) today, the Catholic left was an active branch of the faith in the ’60s and ’70s that was perhaps best known for its non-violent protests against the Vietnam War. The Camden 28, Anthony Giacchino’s fascinating documentary about one such devotional group of protestors, is a bit of a slow burn: Fond, stinging, and finally instructive, the film assembles a comprehensive look back at the actions, arrest, and prosecution of a group of political malcontents (most of them young Catholics and some of them priests) in the summer of 1971. Using recent interviews and 2002 reunion footage, The Camden 28 builds gradually, its aging subjects initially recalling their plan to remove and destroy draft documents from a Camden FBI building with something of a twinkle in their eye for trouble-making times past. But when a Judas surfaces in their ranks, and they are suddenly facing 47-year sentences and the wrath (and ethical jujitsu) of J. Edgar Hoover, the story begins to move in surprising—even shocking—directions. The ensuing trial, its revelations, and the integrity of the jury bear out what the true believers who raged against the Vietnam War already knew, and what Americans today seem to have forgotten: That civil disobedience is not only at the core of democracy, it’s a moral imperative.