Stephen Adly Guirgis may be the most extravagantly talented, maddeningly wayward playwright in America. You would not believe the speeches this man writes. People say he has a good ear for dialogue, but that doesn’t explain why his soliloquies are such wonders: precise and evocative, with a raucous theatrical energy. Mr. Guirgis’s lines – I mean this as a compliment – don’t sound as if a playwright wrote them.

Mr. Guirgis’s plays don’t sound as if a playwright wrote them either, and that’s no compliment at all. “The Last Days of Judas Iscariot,” like its predecessor, “Our Lady of 121st Street,” contains so much vivid, lovely writing – great gobs of material that’s funny or sad or moving – that you can’t believe how unsatisfying it turns out to be.

Opening last night at the Public, the play (a co-production with the Labyrinth Theater Company) is the latest installment in Catholic Cosmology Month around Off-Broadway. The action mostly takes place in a courtroom in Purgatory, where a stern judge presides over “God and the Kingdom of Heaven and Earth versus Judas Iscariot.” Mr. Guirgis, who was raised Catholic, wants to know why God’s infinite mercy does not extend to the man who betrayed Christ.

In this version of the story, Judas got a bad rap. He tried to give back the 30 pieces of silver, he doesn’t bear all or even most of the guilt for Jesus’s death, yet he still must spend eternity in despair. That pain is made thrillingly real by Sam Rockwell, whose Judas is the centerpiece of the show. Many varieties of agony pass across his bearded face throughout the evening. He’s the rare movie personality whose work grows subtler, more compelling, on the stage.

Mr. Rockwell owes a lot to Mr. Guirgis: gratitude for the terrific scenes he gets to play, and agitation for all the scenes he doesn’t. The play works best when it shows us life as it might have been lived by Jesus and his disciples. Language is key to what Mr. Guirgis achieves here. To put it clinically, he is a master of American urban vernacular; to put it as one of his characters might put it, the s– is real. In “Our Lady” and other plays, the multihued Labyrinth actors capture the way New York really looks and sounds. Here, Mr. Guirgis makes figures from the Gospel come alive as contemporary, working-class folks.

The slang-ification sometimes goes too far, as when St. Monica (Elizabeth Rodriguez) makes the florid declaration that her son, St. Augustine, is “a father up in this mothaf-ah, a father of the church, got a plaque and everything!” But maybe the best moment in the show is a soliloquy by Saint Peter along the same lines. “My name is Peter,” begins the sensational Craig “muMs” Grant, head bent over a fishing net before him. “They got a basilica named after me in Rome, which is ironic, cuz, back in the day, if you even said the word ‘Rome’ in my presence – more than likely I’d a beat you with my stick.”

Mr. Guirgis captures the ebb and flow of how people speak – for in stance, a charged barroom confrontation between Judas and Satan (Eric Bogosian in goatee, couture, and sneer). It’s clear that Mr. Guirgis has been wrestling with the questions of faith treated here. Judas’s mother (Deborah Rush) announces what’s at stake in the first three minutes of the play: “If my son is in Hell, then there is no Heaven – because if my son sits in Hell, there is no God.”

All the pressure blows this play to pieces. Mr. Guirgis starts with a cliched setup (courtroom presided over by a hardass judge), and compounds his trouble with a lot of banal name dropping (Hegel is quoted, Freud is called to testify). The occasional sophomoric joke is unworthy of the play’s best scenes, as when Satan complains about the effects of a debauched night: “I’ll tell ya – I could barely make it through my double session pilates this morning.”

Now and then the action shifts to Mr. Rockwell, or to some riveting soliloquy, but always returns, inexorably, to the much weaker courtroom scenes. The impassioned, multiracial Fabiana Aziza Cunningham (Callie Thorne) defends Judas. She stands in for the side of the playwright that demands an explanation for God’s lack of mercy. Yusef El-Fayoumy (Yul Vazquez) thinks the punishment only fair. The attorneys have nothing like the reality of some other characters, but the inspired Mr. Vazquez at least supplies comic relief. With his wild gestures and shaky grasp of English, he’s like a cabbie who slipped past the bar exam.

The testimonies, the arguments, the debating of arcane questions of philosophy: All of this puts a drag on the 160 minute show. Some of the witnesses (Pontius Pilate, Caiaphas the Elder) aren’t worth the time lavished on them. Particularly during act two, the play wanders away from Judas’s plight to fumble with larger questions of faith and belief. When is Sam Rockwell coming back, I kept wondering. Finally he did come back, for a moving encounter with Jesus (John Ortiz). Just when the play seemed to have regained its stride, it was over.

“Our Lady of 121st Street,” which played Off-Broadway two seasons ago, featured some of the clumsiest staging I can remember. Here, Philip Seymour Hoffman is a director transformed. As before, his work with the cast is very fine: It’s a credit to him, and to Labyrinth’s acting ensemble, that the performances are so impressive, despite the company missing two of its best actors (Portia and Ron Cephas Jones) and a third (Liza Colon-Zayas) not having enough to do. Mr. Hoffman is also in complete command of the space. He divides the action between the floor area in Martinson Hall, the balcony overhead, and even the rafters way up by the ceiling. (The inventive scenery is by Andromache Chalfant.)

I don’t pretend to know how Mr. Guirgis went about writing this play. Still, it’s not only because profiles often say that he writes a lot, quickly, at the last minute, that the show seems to have been finished in haste. There’s so much to admire in this play’s writing and acting, and so little to hold it together. It is halfway to being a major achievement.

Until April 3 (425 Lafayette Street, 212-239-6200).