Hans Küng, a liberal Catholic theologian whose teachings and best-selling books called for reforms in the church and whose criticism of the idea of papal infallibility led to a formal censure by the Vatican, died April 6 at his home in Tübingen, Germany, where he had been a longtime professor. He was 93.

The death was announced by the Global Ethic Foundation, which Dr. Küng founded in 1995. He had Parkinson’s disease, but the immediate cause was not disclosed.

Dr. Küng, an ordained priest who worked primarily as a scholar and writer, was regarded by admirers and detractors alike as one of the most important Catholic thinkers of the past century. One of his early books helped provide the intellectual framework for some of the reforms instituted at the Second Vatican Council (sometimes called Vatican II) under Pope John XXIII.

Dr. Küng was among the youngest theologians at Vatican II, which ran from 1962 to 1965, and he cultivated a worldwide reputation as an articulate critic, fluent in six languages, of what he considered the church’s failure to adapt to modern times. During a 1963 speaking tour of the United States, he was invited to the White House by President John F. Kennedy, the nation’s first Catholic president, but he was banned from appearing at Catholic University in Washington.

It wasn’t Catholicism that he opposed, Dr. Küng said, but Roman Catholicism — namely, what he viewed as an insular, self-reinforcing Vatican bureaucracy that amounted to an authoritarian regime. For centuries, he said, the Vatican had neglected its spiritual mission as it pursued the accumulation of power and wealth, with the pope reigning as an absolute monarch.

“Are not the resemblances between the Communist and Catholic systems striking?” he said. “Are not both absolutist, centralist, totalitarian, in short, enemies of human freedom?”

(He also likened the church’s pattern of clerical obedience to the lockstep thinking of military leaders in Nazi Germany.)

Dr. Küng believed priests should be allowed to marry and that popes should be elected not by a secret vote of the College of Cardinals but by ordinary priests and church members. He advocated equal rights for women and said birth control, which the church opposes, should be a matter of individual conscience.

His ideas were so revolutionary that many saw them as tantamount to a call for a second Reformation. Indeed, some scholars viewed Dr. Küng as the most serious threat to the Catholic Church since Martin Luther, the 16th-century German theologian whose criticism of the papacy led to a schism that gave rise to the Protestant Reformation. (Wags even referred to Dr. Küng as “Martin Luther Küng.”)

Still, he never renounced his Catholic faith, and the church never removed him from the priesthood.

“I affirm the papacy for the Catholic Church,” he wrote in “The Catholic Church: A Short History” (2001), “but at the same time indefatigably call for a radical reform of it in accordance with the criterion of the gospel.”

Dr. Küng recommended a decentralized Catholic Church, with the pope and cardinals stripped of their role as the sole interpreters of ecclesiastical doctrine. He maintained that the first allegiance of a Catholic was to the example of Christ, not to the church’s hierarchy and what he considered its capricious and outmoded rules.

“Many Christians are saying, ‘Jesus, yes; the church, no!’ ” he said in a 1977 speech at the University of Notre Dame.

In his dozens of books, Dr. Küng examined Catholicism through the lens of other religious traditions. He used historical analysis to question the virgin birth of Jesus and to show that the Catholic Church had existed for 1,000 years before celibacy became mandatory for priests.

His 1970 book “Infallible?: An Inquiry” (published in English in 1971) attacked the doctrine of papal infallibility: the notion that when the pope speaks on matters of faith and morality, he is inerrantly correct.

Dr. Küng noted that the doctrine had been adopted only in 1870 and that there were numerous historical examples of popes making foolish, ignorant and morally dubious decisions.

In a review in the New York Times, theological scholar Martin Marty called Dr. Küng’s study “a reverent book by a disappointed man who urges that Catholicism live not by the propositions of the church but by Christ’s gospel.”

The Vatican’s Sacred Congregation for the Doctrine of the Faith — the body that determines orthodoxy in the church — told the dissident priest to stop espousing what it called his “mistaken views” as a professor of Catholic theology. Dr. Küng refused, denouncing what he called a “Roman inquisition” against him.

In 1979, after John Paul II was named pope, the Vatican declared that Dr. Küng had to give up his position at the University of Tübingen. (It is a state university, but professors of Catholic theology had to be approved by the church.)

By then, Dr. Küng was a best-selling author with a large following among liberal-minded priests and church members. Students held demonstrations against the Vatican’s decision, and hundreds of Protestant and Catholic clerics signed letters of protest.

Dr. Küng moved to a different department at Tübingen as a professor of ecumenical theology and as director of the university’s Institute for Ecumenical Research.

One of the Vatican officials who took part in the decision was Joseph Ratzinger, a German priest Dr. Küng had helped recruit to the Tübingen faculty. But as Dr. Küng held on to his liberal religious views, Ratzinger grew more conservative. He rose in the church hierarchy, becoming head of the Congregation for the Doctrine of the Faith. In 2005, after the death of John Paul II, he became Pope Benedict XVI.

Dr. Küng, who had criticized John Paul II’s leadership, had a cordial discussion with Benedict in 2005, but the church’s stance toward the theologian did not soften. Dr. Küng became a strong critic of his onetime colleague.

“I think it’s very important that we do not sink into pessimism,” Dr. Küng told the German magazine Der Spiegel in 2011. “But my diagnosis has shown that the church is sick, and it’s the sickness of the Roman system. Under these circumstances, I can’t just behave like an ineffective doctor and say that everything will be fine.”

Hans Küng was born March 19, 1928, in Sursee, Switzerland. His father had a successful shoe business, and his mother was a homemaker. He had five younger sisters.

He was 11 when he decided to become a priest. He studied at the Pontifical Gregorian University in Rome, was ordained in 1954 and received a doctorate in theology in 1957 from the Catholic Institute of Paris. He also studied in Germany, England, Spain and the Netherlands.

Dr. Küng served as a priest in Switzerland in the late 1950s before beginning his academic career. He joined the faculty at the University of Tübingen in 1960.

One of his more popular books, “On Being a Christian” (1974), sold more than 200,000 copies in Germany. Another, “Does God Exist?” (1978), wrestled with the most fundamental question of religion. Dr. Küng’s conclusion was that no one could prove or disprove God’s existence.

His other books covered such subjects as music, Judaism, death and dying, modern art, psychology, racism, economics and the environment. Some of his writings have been adapted for a symphonic work by British composer Jonathan Harvey.

After retiring as a professor in 1995, Dr. Küng established the Global Ethic Foundation, which seeks to promote understanding across cultures and religions. His admirers included former secretary of state Henry Kissinger, former United Nations secretary general Kofi Annan and Nobel Peace Prize laureate Desmond Tutu.

A list of survivors could not be confirmed.

When Argentine Jesuit Jorge Mario Bergoglio became Pope Francis in 2013, Dr. Küng hoped for a new era of liberalism in the Catholic Church, but he was disappointed that significant change was slow in coming.

He had long argued that the church was a human institution and that priests and popes were subject to the same flaws and failings as all of mankind.

“There are no unreformable areas of the Church,” Dr. Küng once said, “because the divine and immutable is nowhere except embodied in the human and mutable.”