The obvious allusion in the title of Ross Douthat’s new book, “Bad Religion,” is to a veteran Los Angeles punk band. But I kept thinking of Dan Aykroyd’s “Saturday Night Live” character Leonard Pinth-Garnell, a pompous fop who hosted sketches like “Bad Cinema” and “Bad Ballet.” Pinth-Garnell would introduce a ridiculous performance — think of John Belushi attempting arabesques — then sum up merrily at the end: “Well now! That wasn’t very good at all, was it?”

Mr. Douthat, a Catholic conservative and an Op-Ed columnist for The New York Times, has written a book about contemporary American Christianity that is quite good. But the religion he describes is comically bad. On the left, he maintains, American Christianity is beholden to a self-centered, Oprah-fied spirituality, and, on the right, Christianity is too often represented by a jingoistic, wealth-obsessed evangelicalism. Mainline Protestantism is disappearing, and a beleaguered Catholicism is running out of priests. (The author ignores Jews and other non-Christians, who should be grateful to slip his noose.)

After World War II, Mr. Douthat argues, churches created a sane, centrist religious culture, anchored by Christian tradition but hopeful about progress. Church attendance reached an all-time high. Suspicions lingered between Catholics and Protestants, and among Protestant denominations, but their larger aim transcended those differences.

“Both doubters and believers have benefited from the role that institutional Christianity has traditionally played in our national life,” Mr. Douthat writes, referring to the Eisenhower era. He cites the old Christianity’s “communal role, as a driver of assimilation and a guarantor of social peace, and its prophetic role, as a curb against our national excesses and a constant reminder of our national ideals.”