Pope Paul VI, who sought to promote the renewal of the Roman Catholic Church while maintaining the unique nature of papal authority, died yesterday at his summer residence at Castel Gandolfo, Italy, following a heart attack. He was 80.

Paul's 15 years as the 262nd successor to St. Peter followed the papacy of the easygoing and earthy John XXIII. Paul's initial work involved maintaining the momentum of Vatican II, the vehicle for aggiornainento - or updating - initiated by Pope John, yet holding to the traditional teachings of Catholicism. Few other popes in history - and certainly none in modern times - have faced such a difficult and baffling task.

Partly because of the seeming contradictions of his work and partly because of his nature, Paul was known throughout his years of "serving God by serving God's people" as an enigmatic personality impossible to classify. Paul was shaped by Pope Pius XII, a refined intellectual, but he was influenced by John XXIII, an offhanded charmer.

But looming over all else was the weight of 20 centuries of tradition.One of Paul's teachers and later a colleague with the Vatican, the late Giulio Cardinal Bevilacqua, summed up the responsibilities of Paul: "He (had) to do all at once what his predecessors did successively."

In the interests of openness, Paul became the most traveled pontiff in history, visiting 16 countries on six continents. It followed that he was one of the most accessible of popes. He granted audiences frequently, and Americans he received ranged from the Rev. Dr. Martin Luther King Jr. to Betty Friedan. To the world, he regularly issued ringing calls for social and economic justice. He opened new lines of communication to communists down the street in Rome's city hall and to those in the Kremlin.

In the name of tradition, Paul maintained the church's position against artificial means of birth control. Sometimes he spoke with deep misgivings about the direction of affairs within the church and deplored the dust storms of criticism that swirled around his actions.

Last year, the editors of Commonweal, the lay-edited New York magazine, said Paul should resign. The right-wing Rome newspaper II Tempo fumed in 1971 that the pope had no business granting a private audience to a group of rock musicians.

The criticisms and attacks may be seen as the inevitable result when an activist leads an organization so immense and so seemingly changeless as the far-flung Catholic Church.

Paul's activism was stunningly apparent in the early days of his service. In January 1964 he left Rome for a three-day visit to the Holy Land. On the Mount of Olives, he met Patriarch Athenagoras, leader of Greek orthodoxy. The pair exchanged the "kiss of peace," with Paul saying the union between the two bodies "should never have been broken." In Jerusalem, he called out "Shalom, Shalom" to festive crowds that followed him. Later, Paul visited Athenagoras in Istanbul and Athenagoras visited Rome.

In the same vein of unity, the pope met with Michael Ramsey, the Archbishop of Canterbury and leader of th Anglican Church in 1966. The two called each other "brother in Christ." They also set up a joint body to explore ways that might lead to the joining of their two churches.

In December, 1964, Paul travelled to Bombay, India. He found himself welcomed as few other visitors ever have been. Some 2.5 million people cheered his presence in what was said to be the largest gathering ever to attend a public event in India.

By then, it no longer was unusual for the pope to pack his bags and fly around the world. His October 1965 trip to New York was a mix of drama, friendship and spirituality. He went before the United Nations and called out, "No more war - war never again." Although this was not a renunciation of the just-war theory that Catholic pacifists such as Dorothy Day long have been urging, it was a stark enough denunciation of militarism to bolster what was then the beginning of the Vietnam antiwar movement in America. By the nature of things, the U.N. speech may be less remembered in the public's mind than the visually rich secene of the pope celebrating a pontifical mass in Yankee Stadium, packed to the highest rafters.

After New York, Paul went to Fatima, Portugan (May 1967); Bagota, Columbia (August 1968); Geneva, Switzerland (June 1969); Kampala, Uganda (July-August, 1969), and on an eight-country tour in Asia and Australia (November 1970).

This unprecedented exposure to the world and the times led Paul into areas of leadership traditionally not considered religious but which, he insisted, should be the essence of religion - peace among nations. He explained his philosophy of outreach on returning to Rome from his trip to New York and the United Nations.

"I quoted St. Paul in my talk, but I did not have to evangelize," he said. "The talk I gave was situated on an other plane - if I dare say so, the plane of Socrates, I was looking for what was reasonable and just, equitable and salutary, what every responsible man ought to think. If I evangelized, it was to preach that Gospel virtually contained within the Gospel, which is the Gospel of reason and justice."

This theme appeared repeatedly in Paul's public statements. In 1965, on the 20th anniversary of the atomic bombing of Hiroshima, he said, "Let this terrible art, which consists in manufacturing, multiplying and storing bombs to terrorize the people . . . be outlawed . . . Let us pray that this murderous device does not kill peace while seeking it."

The pope rejected arguments that stopping the arms race would mean a loss of jobs and closure of factories. In 1972, he told the diplomatic corps that "it is unthinkable that no other work can be found for hundreds of thousands of workers than the production of instruments of death."

Paul's concerns about disarmament were not separate from his larger social goals. They fit with the thinking contained in "Populorum Progressio" (On the Progress of Peoples), the March 1967 encyclical considered by many ecclesiastical analysts to be Paul's most powerful document. If the world has its victims of war, the pope said, it also had its victims of a dehumanizing value system that allowed rich countries to exploit poor countries.

In discussing modern economics, Paul argued that "private property does not constitute for anyone an absolute and unconditioned right. No one is justified in keeping for his exclusive use what he does not need, when others lack necessities."

The pope's thinking uplifted many social activists living under juntas in such areas as Latin America. Suddenly, they saw Paul as an ally in struggles to overcome the problems of landlessness among the poor.

The pope was explicit on this point: "If certain landed estates impeded the general prosperity because they are extensive, unused or poorly used, or because they bring hardship to peoples or are detrimental to the interests of the country, the common good sometimes demands their exproporiation . . . We must make haste; too many are suffering, and the distance is growing that separates the progress of some and the stagnation, not to say the regression, of others."

Not surprisingly, the encyclical provoked a strong reaction . . . The Wall Street Journal called it "warmed-over socialism." Time magazine said it "had the strident tone of an early 20th century Marxist polemic."

The controversy stirred by Paul's views on economics and development was minor compared with the clamor that followed his sixth encyclical, "Humanae Vitae" (Of Human Life), published in July 1968. In denouncing the birth-control pill, as well as other forms of artificial contraception, the pope sought to end a long-festering debate.

But the opposite happened. Not only did the debate within the church increase - as a small army of theologians, bishops and scholars argued that the pope was, at best, misguided - but those outside the church suddenly saw a dashing of hope that the Vatican finally understood the urgency of population control.

The fireworks about the birth-control issue have not ended. In the April 1978 issue of Human Behavior magazine, a Princeton University social scientist reported "that almost all Catholic women married after 1966-70 will have abandoned church teaching on birth-control by the time they have been married 10 years."

Paul never retreated on the issue. Within the church, a powerful faction of supporters argued that, precisely because of "Humane Vitae," history will regard Paul as one of the greatest popes. In such countries as Ireland, where contraceptives cannot legally be manufactured but must be obtained through the mail or imported in some other way, the pope was seen as a defender of sexual purity in an age obsessed with pleasure and heedless of the commandments against sexual license.

Although stung by the critical reaction to the encyclical, Paul avoided striking back at his attackers.In the name of papal authority, he could have silenced the disagreeing theologians, as the Vatican had done in years past.

His attitude toward the dissent is revealed in his remarks in the 1968 book, "The Pope Speaks," a papal dialogue with French philosopher Jean Guitton. "One must not seek to conquer his questioner, but to convince him. In a sane and holy discussion there is no 'master' and no 'slave,' to use the language of Marx, but two servants of the truth," Paul told Guitton.

The man who spoke those words was born Sept. 26, 1897, in Concessio, a village in northern Italy. The son of a lawyer, journalist and politician, he was named Giovanni Battista Enrico Antonio Maria Montini. One of his two brothers, Ludovici, was to enter politics and serve as a Christian Democrat senator. The other was to become a doctor who would practice in Brescia, the nearby town to which the Montini family moved when Paul was an infant.

His childhood flowed with the kind of piety traditionally associated with rural Eurropean Catholicism - night prayers, catechism, the sacraments and awareness of God's presence in daily events. In his early teens, he studied at a local Jesuit school but later withdrew because of poor health. At 19, he entered the local seminary to study for the priesthood.

The vigor of his faith, as well as gifts for scholarship, caught the eye of his superiors and, in only four years, he was ordained. Instead of being assigned to a parish, as were most of his classmates, young Father Montini went to Rome for further studies at the Gregorian Institute, run by the Jesuits. His exellent work there soon was noticed by the Vatican, which sent him on his first diplomatic mission - to Warsaw in 1923 to work under Poland's apostolic nuncio.

After 18 months, he returned to Rome to work in the Curia, the church bureaucracy that monitors everything from Catholicism's immense financial holdings to the most recent drifts in biblical exegesis. Among the administrators of the Curia, Father Montini stood out because of his interests in literature, music and languages.

After becoming a monsignor, he served as an aide to the Vatican's secretary of state, Euegnio Cardinal Pacelli, who became Pope Pius XII in 1939. The relationship between the two men was close professionally and personally. Their 15 years together ended when Pius XII sent Msgr. Montini to become the Archbishop of Milan, a large and occasionally tumultuous diocese.

After all the years in the corridors of the Curia, Archbishop Montini appeared to relish the pastoral vocation of an active clergyman. He performed the usual works of Christian mercy and charity by visiting prisons, hospitals and orphanages. But he also went out of his way to make contact with the workers of industrial Milan. Once, he told them that "the first to abandon the Christian teachings were not you, the workers, but the great industrial bosses of the last century."

In his 8 1/2 years in Milan, Archbishop Montini made pastoral visitations to 694 parishes, blessed or consecrated 72 churches and erected 32 chapels. He moved in the communist strong-hold of Milan so frequently and was so welcomed there that he became known as "the archbishop of the workers."

If such a style of Christianity struck

Paul's flock as unusual, it was less so for this son of a journalist. Paul's father, Giorgio Montini, for 31 years editor of the daily II Cittadino, had been an ardent champion of Christian humanism and social commitment.

What his fater had been able to advocate in editorials and columns, the son helped advance in daily action. In Milan, Archbishop Montini wrote occasional columns for his diocesan newspaper. A favorite topic was the secretive ways of conservaties in the Curia.

In 1958, Archbishop Montini became the first cardinal named by Pope John XXIII. In the early days of the ecumenical council known as Vatican II, Cardinal Montini became known as one of the progressives who saw clearly the need for the church to open its windows to a breeze of fresh air. By then, Cardinal Montini was emerging as fast as anyone among contenders for the papacy.

John XXIII died June 3, 1963. Sixteen days later, the cardinals of the church were locked into the Sistine Chapel of the Vatican to elect his successor. Cardinal Montini entered the conclave, as the electing body is called, as papabile , one of the favorites. On the morning of June 21, white smoke puffed from the chimney of the chapel, signifying that a successor to John had been chosen. Only four ballots had been taken, and the choice was Cardinal Montini.

He chose the name of Paul because of the special place that St. Paul the Apostle holds in the minds of all Christians and to signify his commitment to the unity of all Christians, not just the 600 million members of the Catholic Church.

As he assumed the burdens and trappings of the papacy, Paul kept his personal life simple. He slept less than seven hours a night, rose early and ate light meals. In his private Vatican apartment, he enjoyed the pleasures of classical music. A staircase from the apartment wound to his personal library on the floor below. A linguist who mastered a half-dozen languages, he relished the novels, essays and plays of such universal authors as Dostoyevsky, Bernanos, Baudelaire, Newman and Shakespeare. He considered the French writer Simone Weil "outstanding." He read all of Norway's Sigrid Undset and Germany's Getrude von LeFort.

In his conversations with Jean Guitton, Paul wryly showed his Italian roots by suggesting that for the French "to produce the equivalent of Dante, the [French] writers Corneille and Racine must be rolled into one." He wondered if even that would be enough.

In his prayer life, Paul preferred the simple ways of the priest to the pageantry of great liturgies. He believed that "the priest is the ultimate poet. He has not only the vocation of feeling with, but of suffering with. His chastity signifies that he does not wish to specialize in any vocation, any special situation, so as to assume unto himself whatever is human radiant and painful, in all situations and states of man. You will tell me it is a hopeless ideal. All true ideals exalt and exasperate."

Vatican II had begun when Paul became pope, and he had to deal with complications that Pope John never had imagined. Paul had to respect the liberty of the assembled church fathers as well as control conflicts that naturally arose between prelates of the East and the West, America and the Third World, Europe and Africa.

He also was aware of the outside world's immense interest in the council, which begun in October 1962 and ended December 1965. Americans, for example, followed the intrigues, debates and politics through lengthy dispatches in newspapers and magazines.

Part of the interest and fascination was the novelty of an open council; previous ones had been mostly closed events. The world was curious and hopeful, wondering whether a settlement of differences among Christians could result in control of other global feuds.

After the close of the councils, Paul said that "many crises were avoided. One of the most visible results was that the council took place without too many upheavals. It was never suspended. It arrived, in some ways, beyond all hopes. It is possible to say that the bishops as a whole set themselves to learn and listen, and many were surprised how in four years their point of view changed and broadened, how they sometimes accepted what before the council they would have judged unacceptable or too rash."

The spirit of adaptability, which differed greatly from earlier councils when stubborn minorities held out in seemingly eternal defiance, stemmed from Paul's example, many bishops said.

Thus, if some of Paul's public statements had a tone of rigidity, he was able in practice to modify them. In 1976, for example, he denounced Italian Catholics who supported the Communist Party in elections. But two years later, when Rome had a communist mayor, the pope had a warm meeting with Mayor Giulio Carlo Argan and said he looked forward with "desire, confidence and advance gratitude" to a working relationship with Rome's communist-socialist administration.

In dealing with worldwide communism, Paul broke with the thinking of his mentor, Paul XII, who wanted little or no negotiation with communist atheists. "Why speak when there is no common languages?" Pius asked. Under Paul, the speaking often was voluble. Archbishop Agostino Casaroli, the pope's foreign minister, has been known as "the Vatican's Kissinger."

Archbishop Cassaroli traveled to Moscow to represent the pope at the signing of the Nuclear Nonproliferation Treaty. Three years after Paul became pope, diplomatic ties were resumed between the Vatican and Yugoslavia. In Poland, where almost 90 percent of the population is listed as Catholic, the government went so far as to send a bouquet of flowers to Stefan Cardinal Wyszynski. In Hungary, the Pope's style of summit diplomacy was effective enough to induce the government in 1974 to allow all vacant bishoprics to be filled.

This activity reflects Paul's statement to the diplomatic corps that "the Church, though in herself and essentially aloof from political action, nevertheless claims a place in the civil world." Currently, the Vatican has formal diplomatic relations with more than 100 nations.

In the spiritual realm, Pope Paul initiated a large number of changes. He ended the rule under which Latin was the only language in which the Mass could be said in the Roman rite of the church. In 1967, he allowed use of jazz and other contemporary and popular music in religious services. He permitted Catholics to eat meat on Fridays.

He canonized two American-born Saints. They were Elizabeth Ann Bayley Seton, Mother Seton, who founded an order of nuns, the Daughters of Charity, in Emmitsburg, Md., in 1809, and John Nepomucene Neumann, a 19th century bishop of Philadelphia. Both were credited with miraculous cures.

Inevitably, there were clashes between some church decisions and governments. Thus, Paul opposed laws to permit abortion and divorce in Italy, as he maintained the traditional position on birth control. His opposition was unavailing and abortion and divorce now are legal in Italy.

In recent years, the pope made his anxiety about the future direction of the church increasingly clear. He issued almost weekly warnings against allowing the momentum of change to overcome what have long been regarded as settled principles.

"Let us open our eyes," he said in 1974. "The church is now, in some aspects, experiencing serious suffering, radical opposition and corrosive dissent . . . The Church is in difficulty." Another time he said, "From the heart of the Church, even among our dearest sons, there arises unrest, intolerance and defections. It is an hour of storm."

Perhaps his gloomiest observation, at least regarding the criticism that followed "Humanae Vitae," the encyclical on birth control, came in 1970 when he lashed out at the press. "There are no longer newspapers that inform, but newspapers that deform," he said.

On the last issue, he eventually was persuaded that part of the Vatican's problems with the press stemmed from the closed ways of the Curia itself. A New York public relations firm was called in to help the Vatican improve relations with the media.

In recent years, the pontiff's pace had slowed considerably. In 1967, he became the first pope to undergo major surgery when his enlarged prostate gland was removed. In addition, he suffered the unrelenting pains of arthritis. In the summer of 1977, he said, "I see myself approaching the hereafter." On his birthday in September 1977, he appeared in public as a gaunt and frail figure. Last March, he suffered an attack of influenza that caused him to cancel most of his Holy Week appearances.

Paul told close friends that he wished to die as a pontiff who had kept the church anchored in a time of great shifting of opinions and attitudes. Such an idea is rooted in the earliest days of Christianity when Christ told his followers, "Upon this rock I will build my church." The "rock" was St. Peter, the first pope.

If, during Paul's years as pope, the rock of the church became more weatherbeaten and perhaps even chipped, it remains securely in place.