It's not easy being a Roman Catholic theologian these days. Trying to explain a centuries-old faith's place in modern times is hard enough. Now some Catholic thinkers worry the Vatican is more concerned with unity than messy debates that can lead to new ideas.

The case of the Rev. Peter Phan is the latest example of the tension between church authorities and Catholic theologians. A 2004 book by Phan, a Georgetown University professor, has come under scrutiny for going beyond the Vatican's comfort zone in suggesting that other religions might have merit.

"Individual theologians can be creative, or they can be irresponsible," said the Rev. James Heft, director of the Institute for Advanced Catholic Studies at the University of Southern California. "The exercise of central authority can be overbearing, or it can be a necessary corrective. So it's a complex situation."

American Catholics and the broader public have good reason to care about what may look like an intramural squabble, Heft said. Theologians often do the thinking that contributes to profound changes in Catholic teaching — on everything from the church's relationship with Jews and other Christians to the role of lay people.

The conflict at the heart of the Phan case, he said, strikes at "one of the major questions of our time, especially in the coming decades: How we can speak of one faith expressed distinctively in a variety of cultures?"

In recent decades, the Vatican has clamped down on theologians who advocate fighting poverty and injustice through the social gospel and liberation theology. More recently, the focus has shifted to the nature of Jesus Christ and salvation, one of the defining concerns of Pope Benedict XVI's papacy and his previous work as a cardinal.

Earlier this year, Benedict released a document reasserting the primacy of the Roman Catholic Church, reiterating themes in the 2000 Vatican document Dominus Iesus. That document states non-Christians are "in a gravely deficient situation in comparison with those who, in the church, have the fullness of the means of salvation."

Phan explored salvation and other themes in his 2004 book, "Being Religious Interreligiously," the focus of the Vatican inquiry. The Vatican's Congregation for the Doctrine of the Faith said the book is "notably confused on a number of points of Catholic doctrine and also contains serious ambiguities," according to the National Catholic Reporter.

Among the chief concerns, said the independent Catholic weekly: that Phan's writings could be interpreted as saying non-Christian faiths "have a positive role in salvation history in their own right, and are not merely a preparation for the Christian Gospel." A committee of U.S. bishops is conducting a separate inquiry into Phan's work.

The increasing diversity of Catholic theologians, Phan among them (he is Vietnamese-American), is greatly influencing the debate about Catholicism's place among other religions, said Terrence Tilley, president-elect of the Catholic Theological Society of America.

"What we have in the last 20 years is a new development," Tilley said. "Discussions of the saving value of other faith traditions had been carried on in a European context by European theologians who had little deep and rich understanding of other religious traditions. Their conversations ran on some pretty clear rails. But the train these days is on a different set of tracks."

Phan has declined to comment on the investigation.

The Rev. Thomas Reese, a senior fellow at the Woodstock Theological Center at Georgetown, said the Vatican too often views the Catholic theologian as working in an echo chamber, repeating back church teachings and documents.

The process of debating theology can be messy, but better to endure the messiness than stifle thought, said Reese, who was forced to resign as editor of America magazine after it published articles challenging church teaching.

"If you knew a company where the executive leadership was not on speaking terms with the research division, would you invest in that company?" Reese said. "That's what we have in the Catholic church today. The hierarchy is very suspicious of the theologians and the theologians are very suspicious of the hierarchy. And that's a very unhealthy situation."

The Rev. Joseph Fessio, a former doctoral student of Pope Benedict XVI whose publishing house is the primary publisher of the pope's writings in English, said the Vatican is neither heavy-handed nor closed-minded in weighing questionable theology. What often fails to be disclosed, he said, is the long process allowing all sides to be heard.

"It's important for theologians to talk to each other, reflect and try to reformulate and understand more deeply what the church's belief is," Fessio said. "But if they move outside the realm of the church as soundly defined, then it's a sign that they have gone beyond their competence as a theologian."