American Roman Catholics have come to terms with the jarring changes the Second Vatican Council brought to their church 25 years ago and have assumed responsibility for their local parish life in ways earlier generations could never have imagined, new studies show.

From distributing holy communion at mass to drawing up the parish budget to offering sanctuary for illegal immigrants, lay men and women today play vital roles in the functioning of their parish. The old-style church, where "Father" made all the decisions and lay Catholics were expected to "pray, pay and obey," has all but disappeared in this country.

These changes, and others that have revolutionized the church in the quarter-century since Vatican II, are being evaluated in Rome where representative prelates from around the world are working with the pope in an extraordinary Synod of Bishops. Whatever the result of this historic meeting, the studies indicate that the changes have permeated and irrevocably transformed the church in America.

There is a new "participatory ethos" among younger, educated Catholics today, said David C. Leege, director of a study conducted by the Institute for Pastoral and Social Ministry at Notre Dame University. Now, he said, "the authority figures have to make the case. They can no longer say, 'This is the church's teaching' and expect obedience. The priest is not the dominant authority anymore."

The Rev. Thomas Gannon, a Jesuit sociologist who directs the Woodstock Center at Georgetown University, says the changes have helped bring about a "do-it-yourself Catholicism, in which a majority of the laity, a large number of [nuns] and a smaller but significant number of priests determine for themselves the terms under which they will affiliate with Catholicism."

Among other changes, the Council launched by Pope John XXIII 25 years ago in an effort to update church teachings authorized mass in the language of the people instead of Latin, endorsed the principle of religious liberty, called for a new openness both to other Christians and to Jews and put a new priority on efforts for social justice.

In an idea that was to revolutionize parishes when its full implications were explored, the Council defined the church as "the whole people of God" -- lay people, priests and bishops all working together. And the Council called for "shared responsibility" for the future of the church.

An extensive nationwide study of Catholic parish life, conducted by the institute at Notre Dame, has documented the extent to which these ideas have penetrated American Catholicism:

*In more than three-fourths of the parishes in the scientifically selected sample studied, lay Catholics were involved in policy-setting parish councils or some comparable form of parish governance.

*Nearly all parishes surveyed had Confraternity of Christian Doctrine (CCD) programs for the religious education of children, and 72 percent had liturgy planning groups -- all staffed and in many cases directed by lay volunteers.

*Among lay Catholics formally enrolled in a parish, the study found that nearly half are involved in one or more parish activities beyond attending mass; that 30 percent of Catholic churchgoers spend an average of five hours a month in parish activities outside of religious rites, with 5 percent devoting more than 25 hours a month.

"In parish after parish we find a cadre of laypersons who volunteer nearly as many hours to parish service as the pastor," said Leege, director of the Notre Dame study.

Today's Catholics expect to be involved. "No parishes are in deeper trouble . . . than those where a bishop has instructed a pastor to pull back on lay involvement in formal liturgical roles and in parish governance," Leege said.

If lay Catholics are taking responsibility for their parish life, they have also stopped relying on what "Father says" about such highly personal matters as sexual morality.

The precise extent of what Gannon calls "do-it-yourself Catholicism" is uncertain, but researchers agree that from 75 to 80 percent of American Catholics in their childbearing years ignore church teaching banning contraception.

Four out of 10 marriages solemnized by the church end in divorce, statistics show. Catholics in the 18-to-30 age range "simply disregard [church teaching on sexual ethics] altogether," said Leege.

For most of its existence in this country, Roman Catholicism was an immigrant church.

Parishes were little islands of Italy, Ireland, Poland, Germany.

Parish priests were both defenders of the faith and defenders of the faithful against the ethnic and religious prejudice of a society in which they were strangers.

The immigrant church began to disappear after World War II, as second and third-generation English-speaking and Americanized children of immigrants joined their neighbors in the flight to the suburbs and Roman Catholics began blending into the mainstream of American pluralism.

Education was the way to catch up, and young Roman Catholics went off to college in unprecedented numbers.

In the mid-1980s, the average level of education of adult non-Hispanic Roman Catholics "is still slightly below that of Jews, Episcopalians and Presbyterians but above that of the large Protestant bodies such as Methodists, Lutherans and Baptists," according to the Notre Dame study. While Catholics are a little over 22 percent of the population, they account for 40 percent of the enrollment in universities and colleges.

Young Catholics are staying in school longer, pursuing higher paying jobs, marrying later and, in a complete reversal of old patterns, having fewer children than Protestants in the same age group, according to the Notre Dame study.

The National Opinion Research Center at the University of Chicago, which has tracked trends in Catholic life for more than two decades, says that only Jews, Presbyterians and Episcopalians have higher family incomes, on average, than Catholics.

Unlike their Protestant counterparts, the higher the education level of young Catholics, the more likely they were to attend mass fairly regularly, the Notre Dame study found.

Among persons of all ages who identify themselves as Catholic, the Notre Dame study found that 44 percent go to mass every week, compared with 38 percent of Protestants who go to church weekly. At the other end of the scale, about 30 percent of Catholics today almost never go to church.

The dropout rate is a cause for concern among Catholic leaders but it also reflects a trend in American life that has hit mainline Protestant churches even harder.

"People are not joining churches as they did in the past," said the Rev. Msgr. Alvin A. Illig, director of the National Catholic Evangelization Association.

Many Catholic scholars agree with priest-sociologist Andrew Greeley, who heads the National Opinion Research Center, that far from driving people away, the reforms of Vatican II made it possible for American Catholics to stay with their church, in light of the cultural and educational transformations they have undergone in the last 50 years.

"It is plausible that the conciliar changes came maybe 10 to 15 years before they were absolutely necessary, and therefore probably averted far worse damage to the American church," said Gannon.

The Notre Dame study found that women account for 57 percent of adult Catholics in the United States and 61 percent of the congregation at Sunday mass.

While feminists, female and male, campaign against papal prohibitions on ordaining women to the priesthood, Catholic women continue to volunteer for every task in the parish that is open to them.

Four out of five CCD teachers are women, the Notre Dame study found; 85 percent of those who prepare the altar for mass are women; 85 percent of those who visit the sick, help the poor and comfort the grieving are women.

But so are 52 percent of the members of the policy-making parish councils, and 60 percent of eucharistic ministers, who help distribute communion, and half of the lectors -- persons who read the scripture lessons at mass -- are women. And efforts by Notre Dame researchers to find the "most influential parishioners," exclusive of the pastor, produced a list that was 58 percent female.

"Women have become visible in liturgical roles that were previously reserved for men, and for ordained men at that," the Notre Dame report noted. "There's no doubt that many women are angry at the slowness" of change by the Vatican on this question, said Leege. "But there are a lot of women willing to be patient a bit longer."

Neither the question of ordaining women nor the church's stand on such issues as birth control and divorce were discussed at Vatican II.

In evaluating what has happened to American Catholicism since Vatican II, Greeley said, "If Catholicism is essentially a religion of sexual ethics, then there has been a decline. If it is, however, a religion about God's love and about concern for the poor and the oppressed, then the evidence suggests that Catholicism as a religion has grown and not declined."