"We're going to see a bunch of seminaries close in the next 50 years," said Gary Hall, the chair of EDS's board of trustees. "The church is shrinking. The need for clergy is shrinking. And the institutional support is shrinking."

Mainline Protestant seminaries are facing an existential crisis after a decade of mounting red ink.

Enrollment has fallen by nearly 25% over the past decade, according to the Association of Theological Schools, an accrediting agency.

Mainline churches, where membership has been falling for decades, can support fewer full-time pastors than in the past. Denominations are pulling back their financial support for seminaries, while the cost of educating students is still going up.

As a result, some of the oldest and most celebrated seminaries in the country--institutions that helped shape both Christianity and higher education in the U.S.--are on the brink of financial collapse.

Andover-Newton Theological School, the nation's first graduate school of any kind, founded in 1807 outside of Boston, sold its campus in July and is moving to the Yale Divinity School campus in Connecticut.

Claremont School of Theology recently said it is hoping to sell its campus and relocate from Southern California to a less expensive area.

Episcopal Divinity School--which pushed the Episcopal church to ordain women and accept gay and lesbian members--will affiliate with Union Theological Seminary in New York. But the school's faculty and students aren't invited to join.

Daniel O. Aleshire, the former executive director of the Association of Theological Schools, said seminaries were facing a "mergers and acquisitions environment." Many smaller schools are joining with larger ones on university campuses, which have remained more financially stable.

Still, Mr. Aleshire didn't see this as the end-times for theological higher education, but as an evolution of its mission. "Theological education is starting to look more like an executive M.B.A., not a degree required for initial practice in ministry," he said.

Until recent years, seminaries largely focused on training young college graduates to become full-time church pastors.

Schools like EDS and Andover-Newton helped establish a template for what being a church pastor in the U.S. meant. Students spent three years on a seminary campus studying for a master's of divinity, the degree many denominations have long required for pastors. They studied Greek and Hebrew so they could read some of the earliest Bible texts.

Such academic rigor gave rise to a class of clergy who were often among the most educated and respected people in their communities.

But as the nation has grown more secular, the role of clergy, and seminaries, has shifted.

With dropping church attendance, there are fewer full-time pastor jobs available. In the past 25 years, the average age of all pastors in the U.S. has risen to 54 from 44, according to a study by the Barna Group, a research firm specializing in the study of religious beliefs.

Seminaries have taken steps to attract more students, effectively redefining theological graduate education in the process.

A shrinking portion of students are pursuing the traditional three-year master's of divinity. Schools are attracting older students, who may not plan on a career in church ministry. A growing portion of seminary students are taking most classes online.

Claremont School of Theology, which traces its roots to 1885, has more than doubled its enrollment since 2010 to more than 400 students, in large part because most of the school's students now take most of their classes online.

The school has also embraced an interreligious curriculum, and only a quarter of the students are members of the United Methodist Church, with which the school is affiliated.

"We take seriously our role in training religious leaders. But we also take seriously our role in training leaders for nonprofit organizations and other institutions," said the Rev. Kah-Jin Jeffrey Kuan, president of Claremont School of Theology.

Lisa Devine, who graduated from EDS in May, is typical of many mainline seminary students these days. At 36, she lived in California most of the time she attended EDS.

She said she doesn't "feel called to parish ministry." Instead, her family plans to start a "therapeutic farm."

"I think theological education needs to change, and in many ways it already is," Ms. Devine said. "No one can take three years out of their life."

Other Christian denominations are confronting similar challenges recruiting and training a new generation of clergy. For Roman Catholics, there is now one priest for every 1,800 Catholics in the country--more than twice what the ratio was in 1970. More than 3,000 parishes in the country now lack a full-time priest.

At evangelical Christian seminaries, enrollment is down slightly over the past decade, though it has rebounded over the past two years. Many of the churches that are growing fastest, like nondenominational Pentecostal churches, don't require a seminary degree for pastors.

Skye Jethani, an evangelical Christian pastor and author, said falling seminary attendance is a symptom of a growing "consumerism" in American Christianity.

"Fewer churches have the expectation that pastors have gone to seminary," Mr. Jethani said. "In popular evangelicalism, they don't really care about your theology. What they care about is, are you an entertaining speaker, or can you run a complicated business like a megachurch?"

Still, some seminaries are growing.

Albert Mohler, president of Southern Baptist Theological Seminary, a theologically conservative school in Louisville, Ky., said the school's enrollment had hit record highs in recent years. A growing number are taking classes online, and a record number are studying to be church pastors.

In an era when Christianity is becoming less dominant, Mr. Mohler said, young people heading to seminary wanted an "unquestionably orthodox theological education."

"A theological seminary that isn't training pastors is just negotiating its way out of business," Mr. Mohler said. "There's nothing other than training pastors that couldn't be done more efficiently and inexpensively by someone else."

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