Under the marble dome of the historic St. Matthew's Cathedral, the Catholic Archdiocese of Washington will ordain six men into the priesthood today. Four are over age 40 -- a retired government librarian, a former House of Representatives staff member, an apartment manager for 20 years and a former Episcopal priest.

Across the country, the pattern is much the same: unmarried or widowed men in midlife, turning to a spiritual career with as much enthusiasm as younger men embrace business and law.

At the same time, fewer men fit the stereotype that the other two priests being ordained represent: men in their twenties straight from the seminary.

About 35 percent of the 3,800 men undergoing seminary training for the priesthood in this country are 31 or older, about five times the proportion 20 years ago, according to research done for the U.S. Catholic Conference.

These men are entering the priesthood toward the end of a tumultuous era in the life of the American Catholic Church, years in which congregations struggled to put into practice the liberalizing changes advocated by the Vatican II council.

Observers of the church expect this older generation of new priests to uphold or even expand one of the most significant of the reforms, a greater leadership role for the laity. At the same time, some experts expect the older priests to resist more controversial challenges to the church hierarchy.

"They will be stable figures in a postconfrontational era," said Eugene Kennedy, Catholic author and psychologist at Loyola University in Chicago.

While the phenomenon of second-career priests has spawned three seminaries in this country for these men, it has raised concerns. "What effect will they have on our retirement fund?" asked the Rev. Robert Sherry, executive director of the U.S. Catholic Conference's committee on vocations. "Will they promote the priestly profession with the youth?"

But for now, officials acknowledged, the Catholic church has little choice but to embrace them, given the continued decline in the number of priests, characterized by two men dying or leaving for every one who is ordained.

Robert P. Green, 57 and former chief librarian for the General Services Administration, said his first notion of becoming a priest came at age 13, when his mother, who was not Catholic, lay dying in a small hospital in Southwest Washington. Running through a heavy December snowstorm in search of a clergyman, he came upon St. Dominic's Catholic Church. A priest, whose name Green never asked, went with him to the hospital and stayed with his mother until she died.

Green converted to Catholicism four months later. Thoughts of the priesthood receded in ensuing years as he turned almost single-mindedly to finishing his education, then to building a career around books.

He won a scholarship to attend Gonzaga College High School, then went into the Army for two years. He put himself through night school at George Washington University, working by day as a librarian at the University of Maryland. In 1960 he went to work for the GSA library; in 1965 he became its chief and oversaw the building of its new facility. "I was serious and happy with my career," Green recalled this week.

What little spare time he had during those years was mostly spent playing with children -- nieces, nephews and godchildren. He never married, though he dated some, "went out to the theater, to dinner, enjoyed a Manhattan or two." He attended church regularly, he said, but "I didn't have much time for church work."

Green said he intended to retire early and take a teaching position in a small college somewhere, "becoming a Mr. Chips kind of person." But beginning in the early 1970s, the notion of religious service began tugging at him again.

Someone gave him an application to become a permanent deacon, an official of the church who can carry out many duties of a priest. He ignored it for three years. "You get caught up in the ordinariness of life," he explained, "and you don't always listen to God's call."

He finally heard that call at a 7 a.m. mass, during the reading of a Bible passage where Jesus calls His disciples to follow. In 1976, he began training for the deaconate; in 1979 he retired from the GSA. Two years ago, he decided to train for the priesthood, "to integrate my secular and spiritual life." His decision was made as life seemed its most fragile: While a deacon, he had a heart attack and two strokes that left him with some numbness on one side. "I decided that if I survived, God still had things for me to do," he said.

The turning point came for James Oberle in 1982, when, at the age of 37, he was considering running for Congress. A PhD gerontologist born in New York, he had had a highly successful career in state and federal government, ending with his staff director's job at the House subcommittee on health and long-term care.

Friends urged him to run, yet he was dogged by the idea that despite his love of the public life, "there was a level of satisfaction I had not reached. I finally said, 'Okay, kiddo, you've got to make a decision.' " He began his priestly training in 1983.

Yet despite their keen interest in the church, his family was stunned, Oberle said, when he telephoned them to say he had decided to study for the priesthood. Because he had been dating regularly, "they thought I was calling to announce my engagement."

At some points during seminary training at the North American College in Rome, Oberle had some "dark nights of the soul." Frustrated in his attempts to understand his courses, which were in Italian; homesick, and occasionally unable to reach his family by phone, he asked more than once, "Is it really worth this?"

The answer came in his second year in seminary, he said, late at night in a small chapel in the Alban Hills beyond Tivoli. While praying, he said, "I became aware that God was more present to me than I was to myself."

Oberle, now 41, talks often about society's disadvantaged in a manner that recalls the intensely political generation of priests ordained 20 years ago in the midst of the civil rights movement and the Vietnam War. "I will preach a social doctrine," he said.

Edward Studney, 66, does not seem to be cut from the same cloth. With characteristic bluntness, he said: "I am not a crusader for causes."

Studney, a program director for WOL radio in the 1940s and 1950s and subsequently an apartment house manager, readily concedes that he considered the priesthood because the church had changed. Following the Vatican II council, the clergy and laity began playing greater leadership roles, he said. He welcomes the fact that today's church "is no longer a monolith."

And yet, the church may have opened too much, he said, for it is "not able to give the answer to everything immediately," and some lay members have assumed roles that "weren't appropriate for them." Certain doors, he continued, "are not open at this time," especially the door to ordination for women. "In some of these areas, we've waited so long," he said, "maybe we should wait a little longer."

Winthrop J. Brainerd, the former Episcopal priest, also wants to wait a little longer before women are ordained. In fact, the Episcopal Church's decision to allow women priests played a part in his decision to exchange one clerical collar for another.

Brainerd, a 47-year-old Canadian who carries himself with military bearing, said he believed that his former denomination was abandoning "its moral and theological positions on just about everything," including abortion. "When you say everything is situational," he said, "you have become God yourself."

His decision was not easy to make, because he had spent years of study at an Anglican boarding school and an Anglican seminary at Oxford University as well as eight years as rector at Christ Church in Baltimore.

"You can't just say, 'Okay, chaps, I've had a revelation, you're wrong and I'm right, bye-bye,' " he said. He resigned in December 1985.

The two young priests being ordained today, John T. Dakes, 27, and Thomas Chleboski, 26, also faced tough questioning, mostly from friends who attended the same Catholic high schools they did.

Dakes, a rakish-looking man with dark hair and mustache, said his girlfriend in his senior year "didn't know what to say" when he told her he wanted to become a priest. He was the only one of his small group at St. John's College High School in Northwest Washington to go into the priesthood, he said; other friends became lawyers, doctors, engineers.

Chleboski, of Stroudsburg, Pa., weathered some of the same questions. "Part of me wanted to marry, have a family, go into politics," he said. Yet this cherub-faced former altar boy could not forget the statement of an Irish priest when Chleboski was about 13. "That boy," the reverend told others in the parish, "is going to be a priest."