While most of the city prepares to welcome Pope Benedict XVI to New York today, one corner of town will offer a tribute that’s decidedly less inviting. The documentary “Constantine’s Sword,” opening today at Lincoln Plaza Cinemas, casts a critical eye on the church’s treatment of other religions, primarily Judaism, since the 11th century, and its findings aren’t pretty. But while the film offers a blistering indictment of papal practices from Hitler’s era through the present day, it fails to resolve either of the major questions it asks: Where did anyone get the idea that killing in the name of God is all right? And what is the origin of the rise in evangelical Christianity in the United States Air Force Academy?

If these seem somewhat unrelated, their unity can be found in the background of the film’s narrator, James Carroll. Mr. Carroll, a Boston Globe columnist who wrote the book from which the movie takes its name, is himself a curious mix of church and state. The son of an FBI agent who later helped establish the U.S. Air Force, Mr. Carroll was raised on military bases and dreamt of joining the Air Force as a boy. But he was also grounded by his church — so strongly, in fact, that he decided to forgo the Air Force in favor of the seminary, and spent five years as a Roman Catholic priest.

“The cross was central to the way I saw the world,” Mr. Carroll says in the film. “It was like a signing device to me. And then, it changed — I began to see that this cross, it throws a shadow.”

Such was the start — ignited by his disgust with the Vietnam War and his religion’s condoning of it — of Mr. Carroll’s disillusionment with the church. Since leaving the priesthood to become a writer, Mr. Carroll has published 10 books, including one novel, which deal primarily with the intersection of religion and war. “Constantine’s Sword” begins at the physical representation of this intersection, Colorado Springs, Colo., where both the Air Force Academy and the epicenter of American evangelicalism, Ted Haggard’s New Life megachurch, are located.

At the academy, Mr. Carroll finds Casey Weinstein, a Jewish Air Force cadet who expresses his displeasure at having found promotional fliers for “The Passion of the Christ” on his dining hall cafeteria tray for several days. He also finds a former chaplain, Melinda Morton, who was dismissed for her refusal to urge non-evangelical Christians to convert, an issue cited as a widespread problem by a study of the academy by the Yale University Divinity School in 2004. And, a few miles down the road, Mr. Carroll finds Mr. Haggard, who, through his permagrin, shares his unique take on freedom of religion: “I always say the same thing: If you want an absolute guarantee of eternal life, Jesus is that guarantee. If you want to try to work it out on your own, you’ve got that freedom.”

How, Mr. Carroll wonders, did the state come to condone a religious movement that trades so heavily in the condemnation of other religions and whose modus operandi is religious conversion? He looks for his answers in Europe, where he traces the Catholic church’s indiscretions, from the massacre of Jews committed by the 11th-century crusader Constantine, to the Spanish Inquisition, to the 16th-century papal edicts that confined Roman Jews to a 4-square-mile ghetto.

It is in this ghetto that Mr. Carroll jumps the narrative back to the present with the Limentanis, a Jewish family that was saved from extermination during World War II by its association with a priest. The Limentanis are glassmakers who have prepared sets of dishes for every pope since Pius IX, and, in the film, are awaiting an order from the then newly elected Benedict XVI.

Mr. Carroll is not at all taken with the current pope, and uses one of the pontiff’s first speeches, in which he spoke about the origin of Nazi anti-Semitism, as an example of what he sees as the spiritual leader’s — and the church’s — selective memory. “He said Nazi hatred of Jews was ‘born of neo-paganism,’ as if that’s the only source of it,” Mr. Carroll says. “Well, it was born of neo-paganism, but that hatred had two parents, and the other one, the long tradition of Christian anti-Judaism, he didn’t mention.”

Mr. Carroll also uses the pope’s invocation of Edith Stein, a Jew who converted to Catholicism, became a nun, and was made a saint in 1998, as another example of his revisionist history. Stein, it turns out, wrote a letter to Pope Pius XI urging him to condemn the Nazis’ anti-Semitism and take action against Hitler’s newfound grasp of government, only to be murdered at Auschwitz-Birkenau in 1942. The papacy, Mr. Carroll points out, has long ignored this critical portion of Stein’s biography.

So how does this all tie together? Well, it doesn’t, really. Mr. Carroll and the film’s director, Oren Jacoby, are endlessly fascinated by the innumerable threads of Christianity’s tapestry, and their limited attention spans make for a fractured and somewhat frustrating narrative.