In Keith Bunin’s new play “The Busy World Is Hushed,” briskly directed by Mark Brokaw at Playwrights Horizons, faith and doubt go mano a mano for two acts. As expected, it’s a draw, but the debates are so inflamed you’d think religious doubt had just been invented.

Mr. Bunin’s excessive earnestness about his play’s grand questions actually distracts his audience from his qualities as a writer. His wonderfully natural ear for comedy and his talent for characterization are nearly buried under all that exalted angst.

The arguing takes place in a home library on West 122nd Street, where an Episcopal minister named Hannah (Jill Clayburgh) is writing a book with the help of her agnostic research assistant, Brandt (Hamish Linklater). They’re interrupted periodically by Hannah’s drifter son Thomas (Luke Macfarlane), an atheist who’s spending a spell at home after stints as a med student, a Benedictine monk, and a yurt-builder.

The neatness of this setup smacks of a senior thesis – you’ve got your believer, your nonbeliever, and your skeptic, each a mouthpiece for the requisite philosophy. Worse, the characters speak in perfectly formed paragraphs; it strains credulity that three such voluble people could exist, let alone all under one roof.

Ms. Clayburgh fares best with this affectation; she convinces you she’s the kind of brilliant scholar who gives flawless extemporaneous sermons. That’s why she needs Brandt – to write down her spoken thoughts and massage them into book form. And she dates from an earlier, more formal era. But in the mouths of Brandt (who’s 30) and Thomas (who’s 26), the play’s chunky speeches sound downright stagy.

Mr. Bunin’s writing nevertheless won me over in the first act. There are plenty of well-deserved laughs for the audience (if few for the straight-faced characters), and there are wonderfully spontaneous moments between the actors, who seem to relish their roles and thrive on the intimacy of the setting.

And the characters are intriguing. Hannah may be a staunch believer, but she’s no ideologue. Her incisive commentary on everything from the authenticity of the Gospels to the place of gays in the church establishes her as a freethinker and questioner – the kind of minister agnostics really like. Her enthusiasm for her subject is contagious. Ms. Clayburgh gives Hannah a concentrated intensity that makes us curious about her: Who is this rail-thin, sharp-tongued dynamo?

She is, as it turns out, a survivor. Twenty-seven years ago her husband drowned (probably intentionally) off the coast of Maine, as she lay nearby, three months pregnant with Thomas. This event, the defining one of her life, seems to have frozen her heart and consolidated her faith.

Now Thomas – the infant who once gave Hannah a reason to go on living – has grown up to be an atheist (yes, a doubting Thomas) and a gay man. The liberal Hannah can live with both; what she can’t face is his restlessness. To her, Thomas’s devil-may-care hiking trips to deserted places have a vaguely suicidal whiff.

For his part, Thomas is consumed with throwing a long-running tantrum. He’s an uncannily authentic 20-something dropout; you feel you’ve met him before, maybe at a youth hostel in Amsterdam. Proud of his enlightened views and ripped clothes, emboldened by a small trust fund, relishing his battle scars from outdoorsy adventures, he is blissfully untroubled by his cliches. For years, he’s enjoyed making Hannah worry about him – it’s his revenge for years of feeling second to Jesus in her affections – but now he wants more than revenge: He’s come home to force Hannah to connect with him.

At first the melancholy Brandt is an awkward bystander in this family drama, lingering longer than an outsider should when the fights get nasty. He sticks around because he romanticizes Hannah’s mind and Thomas’s body, and eventually, he gets some of both. By the end of the first act, Hannah is literally begging Brandt to seduce her son; he’s only too happy to oblige.

Watching two lovers trade naughty glances while Mom perorates about Jesus is already weird and inappropriate, but to make matters more uncomfortable, the ever-more-annoying Thomas decides to drag Brandt into his battles with Hannah. In the ensuing free-for-all, both of them are unapologetically horrible to Brandt, despite the fact that his father is dying of cancer. It turns out that what mother and son really have in common is emotional stinginess.

The second act’s bloated melodrama is enough to make you forget the first act’s warm, wry humor. By the second act everyone is more irrational and less interesting than before; they simply trade arguments, periodically dialing up the volume.

Many a new off-Broadway play tries (and fails) to force its naturalistic characters into some grand, predetermined design. But “The Busy World Is Hushed” has so much going for it – such native liveliness and spirit – that it feels particularly harsh to watch it sink under the weight of its Important Issues. Instinctively, Mr. Bunin can make audiences laugh and think; his play doesn’t need its trumped-up conflict and its intellectual arguments nearly as much as he thinks it does.

Until July 9 (416 W. 42nd Street, 212-279-4200).