“I think maybe I could be religious or whatever if it made any sense, but it doesn’t make any sense. You know?”

A hard-earned state of grace springs from such halting, even banal language in “100 Saints You Should Know,” Kate Fodor’s incisive new drama about two splintered families joined by little more than a desperate need to undo the damage. Aided by director Ethan McSweeny and an accomplished five-person cast led by the superb Jeremy Shamos, Ms. Fodor comes closer than any playwright in recent memory to making sense of modern Christianity.

In a shrewd commingling of intimacy and shame, she begins the play by putting Father Matthew (Mr. Shamos) and Theresa (Janel Moloney) together in, of all places, the bathroom of Matthew’s rectory. The encounter is more innocuous than it sounds: Theresa, a single mother who regards the priest with deep respect despite not being a parishioner, is a cleaning lady assigned to the rectory. The polite but distracted Matthew, who is about to leave the parish under murky circumstances, has no memory of their several prior meetings.

Matthew flees to his doily- and figurine-cluttered childhood home, where his mother, Colleen (the priceless Lois Smith) hovers nervously and assumes the worst about his hasty departure. Theresa, in turn, has to contend with a snappish 16-year-old daughter named Abby (Zoe Kazan), who says things such as “I’m not mad at you. I hate you. Those are two different things.”

For reasons not entirely clear even to her, Theresa drives two hours to Colleen’s home with the pretext of returning a book that Matthew had left behind. Zoe stays outside in the car, then ventures out to antagonize Garrett (Will Rogers), a neighboring boy wrestling with his own impious thoughts. Garrett’s father has warned him to stay away from Matthew, but the tart-tongued Zoe may prove just as risky an influence.

That misplaced book of Matthew’s is “Dark Night of the Soul,” by the 16th-century mystic St. John of the Cross, and nearly all of “100 Saints You Should Know” (the title comes from a different book, one that Theresa read as a girl) takes place on a similarly forbidding night, one in which all five characters’ assumptions about religion and forgiveness come into stark relief.

Abby’s fumbling attempt at belief comes fairly late into that night, in an Act II dialogue between Abby and Matthew that represents perhaps Ms. Fodor’s and Mr. McSweeny’s finest work. The two characters, both chafing at overt concern and (barely) covert disappointment from their respective mothers, are grappling with intense guilt. Ms. Kazan peels back Abby’s protective carapace with touching hesitancy, while Mr. Shamos — whose past credits leaned heavily on period comedy (“The Rivals”) and even musical comedy (“Gutenberg! The Musical!”) — displays a riveting depiction of a man ministering to others while struggling to quash the doubt that has pervaded even his most rudimentary tasks.

This doubt comes to the forefront in a beautiful monologue in which Matthew speaks directly to the audience about the actions that have led to his enforced hiatus. Ms. Fodor offers a haunting description of just how rich and how crippling the sheer otherness of the priesthood can be. “The Church teaches that as priests we are sanctified,” he explains, “that is, literally, separate, set apart for sacred use. I’m set apart. I’m set aside. I’m lonely.”

This is far too threatening an idea for his mother. Colleen is the sort of woman who harangues a guest for a bite of food or a game of Scrabble and then, when the person finally relents, says with a pleased grin, “I thought you might.” But Ms. Fodor also locates the sadder manifestations of Colleen’s indirection, as when she asks Theresa to dispel her fears about her son: “I’m wondering if the people at the church like Mattie, because I worry that he can be difficult.” They do not like Matthew right now, at least not the people who control his fate, and Ms. Smith creates with piercing sadness a woman who has worked very hard for a very long time to not know that this would happen. Ms. Fodor missteps slightly in introducing a tragic development halfway through “100 Saints,” conveniently removing a threat to one central character and nudging several others into unnecessarily tidy epiphanies. (It also involves the leafless tree that serves as the centerpiece of Rachel Hauck’s austere set design.) While clarity can certainly be prompted by such a calamity, it can just as easily come from the ever-present shifts and tugs that define modern life; relying on the former smacks of dramatic shorthand, particularly in a piece that catalogs these lesser moments with such precision. But it’s strictly because Ms. Fodor has established those various dynamics so insightfully that this minor capitulation jumps out. It represents, of all things, a lapse of faith in her finely drawn characters. And as she demonstrates with vigorous wit elsewhere in “100 Saints You Should Know,” faith – or its absence — can take people in shocking directions.

Until September 30 (416 W. 42nd St., between Ninth and Tenth avenues, 212-279-4200).