It is time for holy communion in the crowded Georgetown church. Standing before one of the queues of worshipers, Joan Challinor takes a consecrated wafer from the container she holds and freezes it in the gaze of the suppliant before her. "The body of Christ," she proclaims.

Like tens of thousands of Catholic lay men and women around the world, Challinor regularly participates with the priests in one of the church's most solemn sacraments.

The participation by lay people in the church's ministry, taken for granted now, is just one of the ways that an extraordinary gathering that began in Rome 20 years ago tomorrow changed, and continues to change, the course of church history.

Summoned by Pope John XXIII to "open the windows" of the church to the world, the Second Vatican Council -- Vatican II, as it came to be known -- changed the liturgy from Latin to the language of the people, extended theological olive branches to Protestants, endorsed religious freedom, and sought to modify the authoritarian structure of the church with the concept of "shared responsibility." It offered new definitions of the church's role in society and the individual Catholic's role in the church, and examined the place of the Bible, bishops, priests, lay members and nuns in modern church life.

"I think it really was the event by which the Roman Catholic Church came into the world," was the way Dr. Robert McAfee Brown, a United Presbyterian scholar, summed it up.

The changes mandated by Vatican II were not abstract ones, of concern only to theologians. They have directly affected individual Catholics, changing the way they worship, their relationship to the church and to God.

"I used to think the church belonged to the clergy -- the priests and the nuns. I looked to them for everything in the church," said Challinor, who with her husband, David, is a member of Holy Trinity, widely known as a Vatican II-style parish.

"I now think the church is the lay people," she said. "I am the church . . . When I go out [from mass] I carry the church with me."

An ecumenical council, a conclave of all Catholic bishops throughout the world, from every continent and many cultures, was the vision of Pope John XXIII. Elected in 1958 at age 76 to be a "caretaker pope," he set in motion what was probably the most revolutionary process to grip the church since Martin Luther.

The sessions that began in 1962 stretched over four years, with 2,860 prelates taking part directly and several times that number attending as advisers or special observers. John, who died in 1963, lived to see only the first session. But his successor, Pope Paul VI, saw to it that the process of aggiornamento. or updating of the church, which John had envisioned, was carried forward.

Some of the reforms have become so entrenched in the last 20 years that it is hard to remember a time when they were considered revolutionary. Others are still being digested and tested, their full implications still unknown.

For ordinary Catholics, the greatest impact of Vatican II was the change in the liturgy: from Latin to the language of the people, with the priests facing their people, and with lay persons participating as readers and commentators and assisting in distribution of communion.

"The changes have given us the kind of flexibility that makes the worship experience when we come together more meaningful," said the Rev. Joseph Byron, pastor of St. Rose of Lima Church in Gaithersburg, who believes the changes in the liturgy have contributed to a stronger feeling of community among church members and have expanded social consciousness.

With the changes, Byron said, "people come because they are more enthusiastic." At the same time, he acknowledged that there is "some nostalgia" for the old-style Latin masses, and "not all of it is on the part of people who actually experienced those ways."

Some Catholics have left the church rather than accept the changes. The most durable movement of dissidents has coalesced around French Archbishop Marcel Lefebvre, who has set up his own seminaries to train priests in the old ways. Pope Paul VI in 1976 suspended Lefebvre's priestly faculties, and the priests he trains are not recognized by Rome.

The council's introduction of collegiality, the concept of church leaders consulting together in decision-making, rather than relying on a single authority, has had far-reaching effects in the church.

Msgr. George Higgins, veteran American Catholic activist, characterized collegiality as "the obvious difference between the way Archbishop Joe Bernardin is going to run Chicago and the way [the late Cardinal John] Cody ran it. . . because they grew up in different generations."

The Second Vatican Council's call for collegiality gave rise to new organizations at various levels of church government. Pastors were advised to form parish councils and bishops to create priests' senates and pastoral councils. Conferences of bishops were organized on the national level, and Pope Paul VI, at the end of Vatican II, initiated a worldwide synod of bishops, meeting every two or three years, to share in decision-making for the whole church.

While Vatican II encouraged collegiality, it established no hard-and-fast guidelines for implementing it. Bishops still have total authority within their own dioceses; it is up to them how seriously they take the consultative bodies. Clashes between advisory bodies, quoting the documents of Vatican II, and bishops, standing on their canonical rights, have not been infrequent.

"Those are things that are going to take time to develop," Higgins said. "Over the long, long haul, the emphasis on collegiality will have long-range effects on the church."

Vatican II's impact, however, has not been limited to the internal workings of the church. "At the opening of the second session, Pope Paul VI said he wanted the council to construct a bridge to the modern world, to move the church out of the besieged fortress mentality," said the Rev. Walter Burghardt, editor of the quarterly, Theological Studies.

The council document, "The Church in the Modern World," laid the groundwork for just such a pastoral approach that "is different from reaching out and telling the world what to do," he said.

Burghardt suggested that this document laid the groundwork for current efforts of the church to "come to grips with nuclear warfare."

Msgr. John Tracy Ellis, a leading Catholic historian who was an adviser at the council, points to the document on religious freedom as "most important for American Catholics." American Catholics, he said, had long been "embarrassed" by the church's "error-has-no-rights" position on church-state relations.

"Non-Catholic Americans could say to us,'Your church teaches a doctrine that undermines the Constitution,' " he said. By adopting the statement on religious liberty, with its endorsement of religious pluralism in church-state affairs, the council inadvertently "put the church's blessing on the American way," Ellis observed.

Catholics and Protestants alike have praised Vatican II for its acknowledgement, for the first time since the Protestant Reformation, that non-Catholic churches are also within the Christian tradition, an opening that resulted in a myriad of ecumenical ventures and alliances. "One can take for granted doing things with Roman Catholics that were not even conceivable 20 years ago," said Brown, a United Presbyterian theologian at the Pacific School of Religion.

Brown, like most other Protestant leaders, feels that ecumenical progress currently "is kind of on hold," with Pope John Paul II giving ecumenicity a lower priority than his predecessors, "but a lot of doors were opened."

Ellis acknowledges questioning about John Paul II's commitment to the reforms of Vatican II. "I don't think it would be right to say he is turning his back on the council," Ellis said, pointing out that the Polish-born pope "made his international image at the council" and pledged, in some of his early remarks as pope, to continue the council's reforms.

For the Catholic Church, like the rest of society, the last two decades have been filled with turmoil, which critics of the council are quick to blame on Vatican II. Church attendance is off sharply, the ranks of priests and nuns have been decimated, eight out of 10 adult Catholics defy their church's ban on artificial birth control, growth in church membership has failed to keep pace with population growth, and bitter controversy over various Vatican II reforms have poisoned relations in more than a few communities.

Church historian Ellis takes a philosophical viewpoint. "With every church council except one there has been this falling away," he said. Of the priests and nuns leaving religious life, he said, "I feel it would have happened anyway because of the nature of our society . . . . The ethos of our national life is so utterly secular."

Burghardt agrees. "I think we would have had the problem of celibacy, problems about the role of women in the church, if Vatican II had never happened," he said. "Vatican II at best can be charged with hastening some of these problems."

Despite all the problems, said Burghardt, "The church is alive. It's bleeding but it's alive." He paused a moment and added, "It started bleeding 2,000 years ago.