For nearly two hours one morning last month, the Most Rev. Thomas Jerome Welsh, bishop of Arlington, sat impassively in the cavernous ballroom of Washington's Capital Hilton Hotel, listening as his fellow Roman Catholic bishops denounced nuclear weapons. When Welsh rose to speak he had a different message.

First he chided the international press corps and his fellow bishops for failing to heed the real threat to world stability: abortion. Then he told those attending the National Conference of Catholic Bishops that peace could best be achieved not through disarmament, but by praying to the Virgin Mary.

To those who know him, it was a familiar refrain. While many members of the Catholic hierarchy are pushing their church to take a more activist stance on issues ranging from nuclear war to Central America, the 61-year-old Welsh has adamantly clung to the orthodoxy of an earlier age. Catholics, he says, should rededicate themselves to the old-fashioned moral values of piety, chastity and obedience and not worry about whether their church is changing with the times.

"The church's real job is to help people live in the world so they can get to a happy eternity with God," he says. "The Catholic Church is always trying to resist a quick fix."

Since 1974, when he was named bishop of the newly created diocese of Arlington, Welsh has shaped his rapidly expanding domain to reflect his traditional views, presiding with magisterial style over 179,000 Catholics in 21 Northern Virginia counties who compose one of the nation's most affluent and sophisticated congregations.

His unflinching conservatism has upset some parishioners who say his legalistic and inflexible approach to church law violates the spirit and reforms instituted by the Second Vatican Council and practiced by many other dioceses. Last month, an Annandale church made an unusual appeal to Rome for help in mediating a longstanding dispute with Welsh, the second such plea a parish in Welsh's diocese has made since his appointment.

The Arlington prelate's traditional views contrast sharply with those of his more liberal counterparts in Richmond and Washington -- Bishop Walter F. Sullivan, who says the church should be "on the cutting edge of society" and who has lobbied for causes ranging from integration to prison reform, and Archbishop James Hickey, a vocal advocate of human rights in Central America.

Welsh has concentrated his energies on building his diocese and fighting abortion, which all bishops oppose but few focus on as intensely. He has led marches on abortion clinics, lobbied on Capitol Hill, and has written scathing letters to politicians he believes have voted wrong on the subject, among them Sen. Edward M. Kennedy, the Massachusetts Democrat who lives in McLean.

He also has sharply curtailed the role of women and laymen in some parishes, banned a catechism containing a chapter on sexuality and contraceptives, and ordered a respected priest working with the poor to leave the diocese because of an unspecified personal dispute. Last fall he made a rare public statement condemning Arlington's cable television system for broadcasting "soft pornography" on its R-rated Playboy channel.

Such actions have endeared him to many conservative Catholics, including military personnel and leaders of the New Right, many of whom live in Northern Virginia.

"He's one of the finest men I ever met," says Paul Weyrich, a founder of the Committee for the Survival of a Free Congress. "I just love him."

His critics, few of whom will speak publicly because of the authority vested in bishops under church law, say his philosophy has resulted in some more liberal Catholics attending church across the Potomac River in Washington.

"He believes in the old 'pray, pay and obey' Catholicism of the l950s," says Ruth Fitzpatrick of Vienna, a parish activist who has tangled with Welsh over the role of women.

"Since Vatican II there's been quite a bit of development about individual conscience, but he just squelches any views that oppose his," says one former priest who has left the diocese for Washington. "He won't even print them in letters to the editor in his diocesan newspaper."

"He's a good person, and he's sincere, but dealing with him is like dealing with the Ayatollah Khomeini," says the Rev. Thomas Quinlan, a controversial and colorful priest whose departure shortly after Welsh's arrival precipitated a rancorous schism at Alexandria's Good Shepherd Church, which was appealed to the Vatican. "He just doesn't listen and if you disagree with him, he looks at you like you're from Mars."

Welsh's detractors concede, however, that the diocese has flourished during his tenure.

In the last seven years, according to diocesan statistics, the number of Catholics has increased from nearly 154,000 to 179,000 and the number of churches from 52 to 56.

The diocese has opened four new schools in recent years with a fifth slated to open next fall in a former Fairfax City high school recently purchased for $3 million. Church officials are also credited with establishing a nationally known resettlement program for Indochinese refugees.

In addition, the church has just bought a 200-acre estate in Washington, Va., for a retreat as well as several properties for religious orders Welsh has lured to Northern Virginia. The bishop has also tapped wealthy, conservative Catholics to help finance Christendom College, a school in Front Royal founded in 1977 as an alternative to Catholic institutions like Georgetown, which some Catholic conservatives consider too liberal.

Welsh, a pleasant-faced and soft-spoken man, is puzzled by the criticism directed against him and pained by charges that he is intolerant of opposing viewpoints. "I consider myself friendly and open," he says.

Welsh lives in an an imposing flagstone home in Arlington valued at $298,000 with another priest, the Rev. Bob Avela, 33.

Avela says Welsh is an avid sports fan who doesn't smoke, watches his weight and drinks only an occasional glass of wine. "He's the kind of guy who can eat just one potato chip," says Avela, a rumpled, bear-like chain smoker.

The third of four children and youngest son of a devout Irish family, Welsh grew up across the street from the church in comfortable, middle-class surroundings in Weatherly, Pa., a gritty coal-mining town of 4,000 where his father was the railroad station manager.

He entered Philadelphia's St. Charles Borromeo Seminary at 15, a decision that did not surprise his family and particularly delighted his mother, according to Welsh's older brother, William. "We always said he was her favorite and there was no higher aspiration in my mother's mind than Tom being a monsignor," recalls his brother, the deputy librarian of Congress.

Welsh earned a doctorate in canon law at Catholic University and then returned to Philadelphia where he worked his way up the ecclesiastical ladder, becoming rector of the seminary and the protege of the powerful and deeply conservative John Cardinal Krol.

In 1974, Arlington was spun off from the Diocese of Richmond after church officials decided Virginia was too large a territory for one prelate. Welsh was appointed to head the new diocese, a move Welsh aides say was designed to mollify conservative priests and influential laymen who were openly rebelling against what they regarded as the excesses perpetrated by liberal Richmond.

For many conservatives, Quinlan personified the problems Welsh was sent to eradicate. Some of his antics -- such as the time he drove a Volkswagen up the aisle of his church because he considered it the modern equivalent of an ass -- are still recalled with horror by some priests in the diocese.

Welsh says he viewed his appointment as "a great opportunity to do anything I wanted." What he did, according to priests who knew him, was try to replicate the more traditional practices of the Philadelphia diocese in Virginia. He ordered his priests to wear black clerical garb, not gray, and replaced Quinlan with an old-style pastor who sharply restricted participation of women and laymen.

When parishioners protested, Welsh staunchly backed the new priest and members of the congregation made what at that time was an unprecedented appeal to the Vatican. "Usually when bishops clamp down like that, people shut up and go away," says one priest. "In this case you have basically conservative people high up in bureaucracies who say, 'Hey, I shouldn't be treated like that.' "

The Vatican appointed another bishop as mediator and a more liberal priest was ultimately sent to Good Shepherd, but not before Welsh received the sort of publicity the church regards as anathema. Some parishioners at Annandale's Holy Spirit Church recently appealed a similar dispute over lay participation to the Apostolic Delegate, the pope's U.S. representative.

In retrospect, Welsh says, "I don't know what would have been a better way to handle it. I prayed over it and I believe the Lord has healed the differences."

His supporters say Welsh is simply enforcing the edicts of the Vatican. "Under the bishop we are doing exactly what the church wants," says Msgr. Richard J. Burke of Holy Spirit Church. "We're not living in the 15th century, just as we are not living in the 25th."

One area in which Welsh believes the church should be active is politics. "It's inevitable," he says. "Politics is the way we do things."

Welsh says he has focused his efforts on opposing abortion in part because of the location and compostion of his diocese, which includes congressmen, White House staffers, lobbyists and generals. "I think we can have much more impact here," he says.

"We're saving snail darters and whales and seals and giant condors and deer at the Smithsonian, strange things as though our conscience was bothering us about killing babies," Welsh continues. "Just compare the number of people who died at Hiroshima to the number of babies who are killed every month."

Because he considers abortion the litmus test for all politicians, Welsh publishes a poll in his weekly diocesan newspaper before every election detailing candidates' positions.

Last September, State Del. John H. Rust Jr., a moderate Fairfax Republican who voted to fund Medicaid abortions in cases of rape, incest or gross fetal deformity, was defeated after leaflets describing him as proabortion were distributed by parishioners at several Catholic churches the Sunday before the election.

"We have always tried to inform people on matters we consider important," says Welsh. "I'm personally realistic enough to know we cannot mandate people voting one way, or voting at all."

He also opposed the Equal Rights Amendment because he doesn't want "women to have equal rights to be tank commanders in the Army." Last July, at the invitation of Phyllis Schlafly, Welsh, who says he admires President Reagan and the Rev. Jerry Falwell for their emphasis on "basic, old-fashioned values," said a special prayer at her gala celebrating the demise of the ERA.

The church, he says, should limit its political involvement to certain areas of competence. He calls the bishops' proposed letter on nuclear weapons "somewhat naive" because he thinks "atheistic communism is still pursuing its avowed goals of world conquest."

"The document gets us involved in sophisticated things," he says. "I don't really know enough about the MX missile, and the military and CIA people you talk to around here see it in a more complex way and understand it better."