The death was confirmed by Nadja Dornis, a spokeswoman for the Global Ethic Foundation, which promotes Dr. Küng’s ideas.

Dr. Küng, who as an 11-year-old Swiss boy knew he wanted to be a priest, stood at the center of Christianity’s great upheavals in the latter half of the 20th century. His relentless challenges to the church hierarchy caused his critics to call him the greatest threat to the church since Martin Luther, even the Antichrist.

As a liberal, he criticized church policy on governance, liturgy, papal infallibility, birth control, priestly celibacy, the ordination of women, mixed marriages, homosexuality, abortion, the meaning of hell and much else.

On some issues, Dr. Küng said, Buddhism and Judaism were more constructive than Catholicism. Serving Jesus Christ is what matters, he insisted — not serving the church that took his name.

Many Catholics supported him, or at least admired his effectiveness. Peter Hebblethwaite, a Vatican expert, wrote that all Dr. Küng’s proposals at the Second Vatican Council were accepted, some in modified form, in the council’s final documents.

“Never again would a theologian have such influence,” he wrote.

As a rigorous, imaginative scholar, Dr. Küng (pronounced kee-UNG) discovered profound similarities between the essential faith of Catholics and that of Protestants, seeming to remove a significant barrier to a historic rapprochement. In his later years he worked to find commonalities in the ethics of all religions as a means toward peace.

His vast writings included books on Thomas More, Freud, Mozart, Jewish views of Jesus, Eastern religions, life after death and the existence of God.

Dr. Küng emerged as a champion of reform in the 1960s at the Second Vatican Council, also known as Vatican II, where he was an official theologian (he was the youngest one there). Pope John XXIII had called the meeting to “let some fresh air into the church.”

Dr. Küng saw the conference as only a beginning. He continued to press for more revisions in church dogma, including ending the ban on birth control and vows of celibacy by priests.

His polarizing effect was evident on a tour of the United States in 1963. The Catholic University of America in Washington forbade him to speak, but he drew thousands of supporters elsewhere. Two years later, Pope Paul VI, John’s successor, responded to the mounting interest in Dr. Küng’s work by inviting him to the Vatican. Paul told Dr. Küng that he would have preferred that he had written nothing, then offered him a Vatican post.

Dr. Küng turned him down. To have taken the position, he wrote later, would have made him a conformist, even though he acknowledged that it might have been “the great opportunity of my life.”

Robert Kaiser, the Time magazine correspondent at Vatican II, wrote in The National Catholic Reporter in 2006, “If he’d played his cards differently, Hans Küng could have been pope.”

But it was unlikely that Dr. Küng could have thought, spoken or acted differently; few could foresee him relenting in his criticisms of the church. In an interview with The New York Times in 1968, he said he saw an equivalence in the Communist and Roman Catholic systems.

“Are not both absolutist, centralist, totalitarian — in short, enemies of freedom?” he asked.

Catholic theologians answered that Christ had entrusted the authentic interpretation of his divine revelation to the church with the apostle Peter, from whom all popes are said to descend. They said that the sort of democratic church Dr. Küng advocated, however meritorious, did not guarantee spiritual truth.

Dr. Küng’s self-confident manner — variously perceived as brilliant, overelaborate or disrespectful — did not always help his cause. One joke had it that he did not want to be pope because then he would not be infallible.

Dr. Küng’s problem, the priest and author Andrew M. Greeley wrote in “The Making of the Popes 1978” (1979) was the envy he aroused among Vatican officials over his popularity and success.

“Other scholars have been re-evaluating the papacy much more quietly — and have said far more radical things than Küng,” Father Greeley wrote.

It was Dr. Küng’s tightly reasoned rejection of the doctrine of papal infallibility in his book “Infallible? An Inquiry” (1970) that led to his dismissal as an official church theologian. He maintained that the doctrine, which was adopted in 1870 and applies only to those extraordinary moments when the pope speaks officially as the vicar of Christ, was not supported by scripture. He gave copious examples of papal mistakes.

In 1979, Pope John Paul II, Paul’s successor, approved the removal of Dr. Küng’s theological authority, meaning he could no longer teach with the church’s sanction or hold any office in church government.

But he was never dismissed as a priest, and losing his theological status turned out to mean only that he was moved to a different part of the same German university. He continued to write popular, well-reviewed, densely researched books, and to draw crowds on lecture tours. Some Catholic thinkers suggested that his new, more independent role made him a better bridge to the secular world.

Soon after Cardinal Joseph Ratzinger, a leader in the campaign against Dr. Küng, became Pope Benedict XVI in 2005, he invited Dr. Küng to his summer residence in Castel Gandolfo, outside Rome. Pope John Paul II had denied more than a dozen of Dr. Küng’s requests for a meeting.

Dr. Küng and Cardinal Ratzinger had become friends when Dr. Küng recruited Cardinal Ratzinger to be a professor at the University of Tübingen in 1965. They split over the student revolts of 1968, which had horrified Cardinal Ratzinger. They continued to diverge, and Dr. Küng came to refer to the cardinal, who was head of the Vatican office responsible for defending church orthodoxy, as “the Grand Inquisitor” or “head of the K.G.B.”

Nevertheless, after Cardinal Ratzinger became pope, the two enjoyed a long dinner at the pope’s summer residence after agreeing not to disagree. Pope Benedict applauded Dr. Küng’s efforts to revive the dialogue between faith and the natural sciences. Dr. Küng praised the pope for reaching out to other religions.

But after Benedict resigned the papacy in 2013, Dr. Küng suggested that the pope had been out of step with “modernity” and that the church was in need of more progressive leadership.

“In this dramatic situation the church needs a pope who’s not living intellectually in the Middle Ages, who doesn’t champion any kind of medieval theology, liturgy or church constitution,” he wrote, “a pope who stands up for the freedom of the church in the world not just by giving sermons but by fighting with words and deeds for freedom and human rights within the church, for theologians, for women, for all Catholics who want to speak the truth openly. ”

Dr. Küng’s image was distinctly nonclerical. Athletic and handsome, he wore crisp business suits, not a priest’s collar, and drove a sports car. On his trips to the United States, he sometimes appeared on television talk shows, and his youthful style drew comparisons to President John F. Kennedy.

Dr. Küng preferred to be called “professor” or “doctor” or “just plain Hans Küng,” explaining that the title “Father” was not traditionally used in German-speaking lands.

Hans Küng was born in Sursee, Switzerland, on March 19, 1928, and named after his father, a prosperous shoe merchant. His mother, Emma (Gut) Küng, had been a farmer’s daughter. Hans had five younger sisters.

Dr. Küng wrote in a memoir, “My Struggle for Freedom” (2002), that his decision at 11 to become a priest reflected his admiration for a friend who had chosen that course. He recalled immediately accepting a celibate life and choosing to no longer sit beside his girlfriend (he had kissed her just once) on the train to school.

After graduating from a public high school in Lucerne, he attended the Pontifical German College in Rome for seven years, followed by more studies there at the Pontifical Gregorian University.

In 1955, to receive his licentiate in theology, he submitted a thesis on the thinking of the great Protestant theologian Karl Barth.

He was ordained a priest in 1954 and went on to do graduate work in London, Amsterdam, Berlin, Madrid and Paris. He earned a doctorate in sacred theology in 1957 at the Catholic Institute of the Sorbonne.

In 1964, Dr. Küng published a more elaborate version of the Barth study as well as an English edition, “Justification: The Doctrine of Karl Barth With a Catholic Reflection.”

With a glowing introduction from Barth himself, the book, which seemed to prove that Barth’s notion of Christian grace tallied with the Catholic notion, was a sensation among both Catholic and Protestant scholars. Both views defend the total freedom and gift of God’s grace, which, they assert, no person earns.

Cardinal Ratzinger wrote in one review, “For such a gift Hans Küng deserves the honest thanks of all who pray and work toward the unity of undivided Christianity.”

Dr. Küng became an assistant professor of dogmatic theology at the University of Münster in Westphalia, Germany, in 1959. The next year he joined the Catholic theological faculty of the University of Tübingen as a full professor.