When Sister Mary Ignatius arrives in a black, floor-length habit with a stern, pursed face staring out from her wimple, you’ll sit up straight. Chances are if you went to parochial school in the 1950s, you’ll probably also be compelled to make the sign of the cross with her. But whether you’re Catholic or not, be prepared to squirm. And if you’re really, really Catholic, just stay home and say a rosary for the cast of Christopher Durang’s ruthless satire “Sister Mary Ignatius Explains It All For You,” which opened over the weekend at The Playhouse in Belfast.

A nasty comedy on the darker days of pre-Vatican II Catholic education, “Sister Mary Ignatius” was Durang’s breakthrough script in 1979. That same year he offended a lot of people, he also won an Obie. “To call my play anti-Catholic because I criticize a nun who’s conservative is akin to saying that `Medea’ is against all mothers,” he said several years later.

Durang’s play is, however, quite vicious in a mostly hysterical way. The setting is Our Lady of Perpetual Sorrows, a parish whose spokesnun glorifully pontificates on heaven, hell, limbo and purgatory (where you can be stationed for 300 to 700 billion years depending on the seriousness of your sins). In addition to sharing Church dogma, she reveals some personal history — that she is one of 26 children in a family which regularly prayed to St. Anthony (the patron saint of lost things) to help them find their mother, and St. Jude (the patron saint of lost causes) to save her from putting her head in the oven.

Quite unexpectedly, Sister is visited by four former students who have come back after 20 years to present a pageant on the life of Jesus. But they have also come to teach her a few lessons about the real life outside Catholic school walls.

“Sister Mary Ignatius,” which runs about the same length as High Mass (that’s just over an hour for you non-Catholics), can be somewhat unruly dramatically and you wouldn’t exactly call it tasteful in spots. But those are just venial sins, and the play, as a whole, is religiously entertaining.

This production, directed humorously by Christopher Bates, also has the benefit of Julie Arnold Lisnet in the title role. She’s scary, and you may want to make a solid act of contrition for laughing so hard at her adept performance. Bless her and Durang, too, for answering one of those niggling questions we all had about nuns. Not “Why does an all-powerful God permit pain and suffering?” but “What exactly DO nuns have up those big old sleeves?”

The capable cast of sinners that visit her include Datvik Deirkrikorian as a lonely single mother, Peter Clain as a cowering alcoholic with a weak bladder, Peter Conant as a gay man, and the way-passionate Laura Graham as a Catholic girl beleaguered by the shock of a random universe. Young Nevin Price-Meader plays a current student who gets cookies for reciting accurately from the Ten Commandments and the Baltimore Catechism.

A fairly blank stage and cartoonish props make up the set for “Sister Mary,” which more than fills up the space with its dense slings and arrows. This is not a show for everyone and it greatly requires a sense of humor, or possibly some residual anger at the misleading teachings that sometimes showed up in Catholic classrooms in the 1950s. But mostly “Sister Mary” is a comedy about the discrepancies between childhood lessons and adult realities.

A final note: Pre-show index cards are provided for audience members to write questions for Sister Mary, so you might want to think one up before you get there. Not to worry if you don’t. It’s not a mortal sin — a topic on which you’ll be an expert by the end of this show.

“Sister Mary Ignatius Explains It All For You” will be performed 8 p.m. Oct. 3-5 at The Playhouse on Church Street in Belfast. For tickets, call 338-3548. The Playhouse has limited seating, and reservations are strongly recommended.