ROME – When I was a precocious sophomore in high school, I once barged into the office of the Capuchin priest who was in charge of our religion curriculum to inform him that I had serious intellectual reservations about the Catholic faith, and I found the answers being supplied in class unsatisfying.

Father Mike Scully, looked at me for a moment with what, looking back, I now recognize as a bemused smirk, and then wheeled his chair across his office and produced a thick book off his shelf: On Being a Christian by Hans Küng.

“Go read this,” Scully told me, “then we’ll talk.”

I didn’t know then that On Being a Christian is regarded as a masterpiece, one of the most daring and provocative explorations ever penned by a Catholic hand on the intelligibility of Christian faith in a modern, secular age. Despite the fact that it’s composed of dense, demanding German academic prose, it became an international best seller.

Handing me the book was, in retrospect, a brilliant ploy to put me off, since what were the odds I’d actually get through all 720 pages? It was also fairly daring, given that Scully knew full well Küng’s work was controversial, regarded by some influential figures in the Church as out-and-out heretical. Bear in mind this was 1981, two years after Küng’s license as a Catholic theologian had been pulled, in one of the first signs that the winds in Catholicism were shifting under the new Pope John Paul II.

For a high school religion teacher to recommend the book to a student thus took no small amount of gumption. Yet in the end the move was perfect, because the effort to make sense of both Küng and his critics was probably the best possible introduction to the contours of an adult faith in the post-Vatican II era.

Many years later, I had the chance to sit in Küng’s living room in Tübingen, Germany, on a reporting trip and tell him that story. He smiled, then began quizzing me to see if I’d actually mastered his ideas. (I didn’t have the nerve to ask how I did, but I didn’t get any invites for guest lectures.)

With the death on Tuesday of the 93-year-old Father Hans Küng – despite all his battles with officialdom, Küng was never stripped of his priesthood – liberal post-Vatican II Catholicism has lost its muse, and the entire Church has lost an original mind.

Born in 1928 in Switzerland, Küng was a theological wunderkind who, in his 30s, published a book titled Kirche in Konzil (in English, The Council, Reform and Reunion) that many observers credit with outlining the agenda for the closing acts of Vatican II. After the council he became the original theologian-celebrity, a lightning rod inside the Church for his liberal positions, including his open rejection of the dogma of papal infallibility, but also widely hailed even in non-Catholic circles for his reputation as a daring intellectual maverick.

At the peak of his fame, how hotly did his star burn? Consider that when he visited the U.S. in 1981, he was actually a guest on “The Phil Donahue Show.”

Küng evoked strong reactions. To admirers, he was the Catholic Solzhenitsyn, a brave dissident speaking truth to power from the ecclesiastical gulag; to detractors, he was a glib snake oil salesman, a traitor who sold out the faith for forty pieces of pop culture silver.

In the popular Catholic mind, Küng likely will be forever linked with the figure who was his erstwhile colleague and, later, his central antagonist and bête noire in the growing divides within Catholicism after the council – Joseph Ratzinger, the future doctrinal czar of the Vatican under John Paul II, who would go on to become Pope Benedict XVI.

During the Second Vatican Council, both Ratzinger and Küng were part of the broad reform majority, seeking to open the intellectual windows and let in some fresh air amid the fossilized neo-scholasticism that had come to dominate Catholic thought and life. In 1966, Küng was serving as the dean of the theology department at Tübingen when the chair in dogmatics became available, and he didn’t bother filling out a terna, or list of three names, insisting instead that his colleague Ratzinger get the job.

The two were a study in contrasts – the flamboyant Küng zipping around town in his Alfa Romeo, the shy and bookish Ratzinger puttering to and fro on his bicycle – but they hit it off. The two men had a standing appointment for dinner every Thursday night to discuss a journal they co-edited, thus making Küng the lone colleague with whom Ratzinger socialized on a regular basis.

Before long things began to sour, as Küng became the captain of the “spirit of Vatican II” constituency in Catholicism, pushing for ever bolder and more sweeping changes in doctrine and practice, while Ratzinger became an important member of the camp worried the baby was being thrown out with the bathwater. In 1971, Ratzinger criticized Küng’s book on infallibility, and in 1976, Ratzinger wrote that On Being a Christian had “handed over [Christian faith] to corruption at its very foundation.”

For his part, Küng clearly carried a long grudge. In 1997, another German Catholic theologian who had often been at odds with Ratzinger, Johann Baptist Metz, celebrated his 70th birthday with a symposium in Ahaus, Germany. Ratzinger was on the program, and the two men spoke fondly of one another. Küng, however, derided Metz for appearing with Ratzinger.

“He is the chief authority of the Inquisitorial office. It’s like having a general conversation about human rights with the head of the KGB,” Küng said in an interview with me at the time.

“This is practically a capitulation to the Roman system, a kind of making peace with Ratzinger, when the real task of political theology should be to identify itself with the suffering people in our church. They are abusing talk about God to avoid dealing with problems in the church.”

It was all a bit much for Metz.

“Sometimes Küng conducts himself like a second magisterium. To tell you the truth, one is enough, at least for me,” Metz said.

That background makes the happy ending to the story especially sweet, since shortly after his election to the papacy, Ratzinger invited his old friend to visit him in the Vatican and the two spent an afternoon in warm reminiscence. The pontiff expressed admiration for Küng’s World Ethics Project and his insistence that there will be no peace in the world until there is peace among religions, and Küng praised Ratzinger’s own commitment to interfaith solidarity.

“It’s clear that we have different positions,” Küng said in a telephone interview from Tübingen afterwards. “But the things we have in common are more fundamental. We are both Christians, both priests in service of the church, and we have great personal respect for one another.”

Looking back, it’s not clear to me that I understood much of On Being a Christian when I first tackled it at 16 – quite honestly, it’s not clear to me how much I understand today. But here’s what I do get: Few figures ever left a bigger imprint on the Church in their time than Hans Küng, few thinkers and writers ever did more to define the contours of debate about the Church, and few had the same ability to make faith seem relevant, perhaps hip and daring, even in the world’s most thoroughly secular milieu.

Upon the news of his death yesterday, the Vatican’s Pontifical Academy for Life tweeted out the following:

“One of the truly great figures in the theology of the last century is gone, whose ideas and analyses should always make the Church, the churches, society and the culture think.”

Indeed, Hans Küng made us think, and probably will for as long as Catholic theology is consumed. Not a bad legacy, that. Requiescat in pace.

Follow John Allen on Twitter at @JohnLAllenJr.