FOR MOST OF AMERICAN history, Catholic bishops could be counted on to bless the banners of every war that came along. Their attitude was epitomized by the comment of the late Cardinal Francis Spellman on the morality of the Vietnam war: "My country, right or wrong. My country."

Catholic youths had trouble getting conscientious objector status during the Vietnam war because as everyone knew, theirs was a "war church." Even within the church itself, the reluctance of the American bishops to speak out on issues before the Second Vatican Council gave them the derisive title of "The Church of Silence."

All that has changed now. Monday a new breed of American Catholic bishops will gather in Chicago for a final go at a long and extraordinarily complex statement that challenges the morality of much of the nation's defense strategy -- their controversial and much debated condemnation of nuclear warfare.

The roots of this about-face -- Catholic journalist Vincent A. Yzermans calls it "the most significant revolution" within the American Catholic Church since its start in 1634 -- lie in church and national politics, and the talent-scouting of one Belgian archbishop.

Perhaps most important is a monumental change in such an historically glacial institution as the Catholic church. Over 60 percent of the hearly 300 American bishops actively serving the church today have been appointed within the last 10 years. And most of them were the choice of the most progressive Vatican representative ever to serve in this country -- Archbishop Jean Jadot.

But to understand that change, like so many of the changes in Catholicism in the last 20 years, one must understand the major factor that was the Second Vatican Council -- specifically a council document titled "The Pastoral Constitution on the Church in the Modern World."

That constitution summons Catholics to take their faith out of their prayer stalls and apply it to secular problems. Priests, nuns and lay Catholics who marched in civil rights demonstrations and who organize the poor against slum landlords -- and bishops who testify before congressional committees on the brutalities of Latin American regimes which seek our support -- all are acting on principles set out in this Vatican II document.

"The Church in the Modern World" specifically condemns the arms race as "one of the greatest curses on the human race" and says of nuclear warfare: "Men of this generation should realize that they will have to render an account of their warlike behavior; the destiny of generations to come depends largely on the decisions they make today."

This constitution offers the logical foundation for the bishops' study of war and peace.

The unusual archbishop, Jean Jadot -- who now holds a high post in the Vatican and is expected to be named a cardinal -- was assigned here in 1973 by Pope Paul VI with instructions to help the American church carry out the reforms of Vatican II, over whose final sessions Paul had presided.

"They wanted to make the American episcopacy younger, more pastoral," explained the Rev. Francis X. Murphy, a veteran Vatican watcher. "They were afraid that if the old guard stayed in control, they would hold back on the implementation of Vatican II and it would do great damage here."

Jadot, a Belgian, is one of the rare non-Italians in the Vatican diplomatic corps. Born in China, where his father was an engineer, he spent much of his life traveling. As a priest, he served in a Brussels parish, as chaplain to Catholic students, as chaplain to Belgian forces in the then-Belgian Congo, and as head of the influential Society for the Propagation of the Faith in Brussels.

In 1968, though he had none of the usual prerequisites, he was tapped for the Vatican diplomatic corps and after only five years of service in Africa and Southeast Asia, was posted to Washington, the most prestigious assignment in the service.

According to Murphy, he was singled out for the U.S. assignment "at the suggestion" of his mentor, now-retired Cardinal Leo Suenens of Brussels, one of the most progressive leaders of Vatican II.

Sauve but approachable, and with a captivating Maurice Chevalier accent, Jadot traveled constantly here, soaking up impressions about Americans and their problems from chance encounters in airport waiting rooms as well as diocesan conference rooms.

With few exceptions, his choices for bishop were markedly younger men, men more open to change; men with a sensitivity to the needs of their communities, to people themselves.

"Jadot bishops," they came to be called. Jadot disliked the phrase. But some of the Jadot bishops had marched in civil rights demonstrations and for other social justice causes. The numbers of black and Hispanic bishops increased. A few of the new bishops even had been missionaries in Latin America, their outlook on life influenced by the poverty and injustices they had encountered there. Others -- like Washington's own Archbishop James A. Hickey -- gained firsthand experience with human-rights violations in field visits to Latin American mission programs they administered from their dioceses.

Whatever their background, the new breed of bishops was less concerned with the ring- kissing and watered-silk vestments that went with the office, and more with getting to know their people. Many moved out of episcopal mansions and into a couple of rooms in a rectory or seminary.

As the Jadot bishops gained in numbers and seniority within the National Conference of Catholic Bishops (NCCB), they took on social justice issues well beyond traditional Catholic causes. They took stands on capital punishment, strip mining, public housing and handgun control.

But it was a traditionally Catholic issue -- abortion -- which led directly to the present pastoral letter on war and peace.

For years, Bishop Thomas Gumbleton of Detroit -- who customarily checks into the YMCA in towns where the bishops were meeting instead of the more expensive commercial hotel -- and a handful of other peace activists among the hierarchy, kept pressing the issue: Why be concerned about the taking of life before birth and so silent on the massive slaughter of nuclear warfare?

In November 1980, after a presidential election campaign with rhetoric about "winnable nuclear wars," the bishops voted to empower a five-man committee, headed by then-Archbishop Joseph L. Benardin, to draft a pastoral letter on war and peace.

NCCB President Archbishop John R. Roach of Minneapolis-St. Paul, put it in context when the pastoral on war and peace was discussed by the full body of bishops for the first time last November: "It is our commitment to the sanctity of life which historically has led us to be advocates of the unborn in the social and political arenas and which now involves us in the necessary task of assessing defense policy in the age of nuclear weapons by the light of moral doctrine."

There is a final reason why Catholic bishops in 1980 could shed their war-church image and challenge the government -- a reason rooted in the dark side of our history.

For much of American history, Catholics were a suspect and persecuted minority, frozen out of the mainstream of national life. In colonial times, Virginia disenfranchised Catholics and banned priests. So did Massachusetts, decreeing life imprisonment for all "Jesuits, priests and popish missionaries." Even Maryland, founded as a Catholic colony, soon outlawed celebration of the Catholic mass. In the early years of our nation, Catholics had equal rights of citizenship in only five of the 13 states.

In the face of this kind of prejudice, Catholics could "earn" their citizenship and prove their patriotism by fighting the nation's wars. In the Mexican War, both sides of the Civil War and the Spanish-American War, Catholics rallied to the colors and their bishops raised no questions about the morality of the conflicts. In World War I, the church established the National Catholic War Council to coordinate war work among Catholics, including the promotion of war loan drives. Even in the Vietnam War, not until 1971 could the bishops bring themselves to call for American withdrawal from that "tragic conflict."

The election of John F. Kennedy in 1960 signaled the end of the most virulent anti-Catholicism in America. Furthermore, today's 50 million American Catholics are no longer a tiny minority but the largest single religious denomination in the country.

"We've gotten away from the immigrant mentality that we once labored under," was the way Archbishop Roach explained it. "Now we don't feel as cautious as we used to about speaking in the public arena."

Msgr. John Tracy Ellis, dean of American Catholic historians, calls the bishops' war and peace pastoral "unprecedented. . . . There's never been anything approaching the reaction that there's been to this." The bishops' attempt to examine the complex issue of national security in the nuclear war through the prism of Catholic morality, is, said Ellis "a most spectacular instance in which the American hierarchy has taken the lead of world Catholicism."

So much for the church of silence.

The bishops' first public debate of their pastoral last fall stirred wide controversy in and out of the church here and abroad. This was precisely what they wanted. The bishops' meeting itself -- usually ignored by all but a handful of reporters -- attracted more media than the papal visit. Organizations of conservative Catholics sprang to life to battle what they saw as the new heresy from their own bishops. In West Germany 26,000 copies of the second draft were sold.

The Reagan administration, alarmed by the challenge from the once-docile churchmen to strategy and weaponry that stands at the heart of national security planning, sent a man to Rome to whip these maverick Americans back in line. The pope responded by awarding a cardinal's hat to Bernardin, the man most publicly identified with the statement.

"Even if we never come to a third and final document," said San Francisco's Archbishop John Quinn at the end of last November's debate, "We have accomplished our purpose. It has raised the moral issue in the midst of the public debate. It has made the point that this is clearly a moral question."

The 150-page document which the bishops will consider over the next two days in Chicago rules as immoral any first use of a nuclear weapon and any use of nuclear weapons against population centers whether as a first strike or in retaliation. In addition, attacks on military or industrial targets "within heavily populated areas could well involve such massive civilian casualties that, in our judgment, such a strike could be deemed morally disproportionate," the statement says.

Possession of nuclear weapons as a deterrent, within the framework of a balance of forces is termed "morally acceptable," as long as such a position is not "an end in itself but as a step on the way toward a progressive disarmament." The statement is taken directly from Pope John Paul II's message at last summer's conference on disarmament at the United Nations.

The pastoral continues: "The moral duty today is to prevent nuclear war from ever occurring and to protect and preserve those key values of justice, freedom and independence which are necessary for personal dignity and national integrity."

The draft of the pastoral calls for "support for immediate, bilateral, verifiable agreements to curb the testing, production and deployment of new nuclear weaons systems," for "negotiated bilateral deep cuts in the arsenals of both superpowers" and for "early and successful conclusion of negotiations of the comprehensive test ban treaty."

The pastoral warns that "if deterrence exists only to prevent the use of nuclear weapons by others," then military planning "for prolonged periods of repeated nuclear strikes and counterstrikes, or 'prevailing' in nuclear war are not acceptable." In addition, the bishops speculate as to whether both the much debated MX and the Pershing 2 missiles might not "be useful only in a first strike" and thus ruled out by their moral reasoning.

Even before the full body of bishops got a chance to debate the third draft of their war and peace pastoral, it was enveloped in controversy: Is the third draft watered down? Have the bishops knuckled under to administration pressures, as some reports claim?

In contrast to widely publicized criticism of the second version last November, administration officials in both the White House and State Department have praised the new third draft. The third draft "explicitly endorses many of the far-reaching objectives the administration seeks," a State Department statement said. The State Department also curiously expressed satisfaction that the bishops' third draft "no longer advocates a nuclear freeze." The bishops point out that neither did the second draft.

The bishops were so annoyed with reports that they had watered down their third draft to suit administration criticisms that they fired off a press release to deny it. In a joint statement, Bernardin and Roach declared that "we could not accept any suggestion that there are relatively few and insignificant differences between U.S. policies and the policices advocated in the pastoral."

The extraordinary efforts of today's bishops to disentangle their moral views from perceived government influence stands in sharp contrast to predecessors such as mid- 19th century Bishop Francis P. Kenrick of Philadelphia, who, while expressing some sympathies for abolishionists, counseled "Nevertheless, since this is the state of things, nothing should be attempted against the law or anything be said or done that would make (slaves) bear their yoke unwillingly."

Will the bishops' pastoral turn all Catholics into conscientious objectors, and force Catholic workers to foresake jobs in bomb factories? No, not likely, although it does admonish those in service as well as in defense industries to evaluate the moral implications of their occupations.

One thing is certain about the pastoral: the Roman Catholic church in this country will never be the same again. And maybe, neither will the country.