But in part it derives from his Swiss assertion of independence and freedom.
He likes to quote Gregory the Great: "Better for scandal to be caused than for the truth to be abandoned."
This stance has brought him, as documented in this second volume of memoirs, continual tension with Rome, ending in a titanic battle.
The book draws out the parallels and contrasts between his course and that of Joseph Ratzinger, now Pope, seen as the last two active representatives of the comrades in arms who fought for renewal at the Second Vatican Council. The divergent currents that followed the council are mirrored in their subsequent careers.
Both were colleagues and professors at the University of Tübingen in Germany. Both had to face the student unrest of 1968, when Ratzinger was struck, says Küng, by "an almost apocalyptic anxiety".
Whereas Küng weathered the storm, Ratzinger felt he had seen how the progressive cause could become a lie, and he now left Tübingen for Regensburg - it was like moving from Harvard to Idaho State University, said some American observers.
He did not understand, wrote a contemporary, that "Küng was fundamentally on his side", and there was a parting of the ways. Yet their different paths continued to cross, and when Ratzinger became Pope, he responded warmly to an overture from his former Tübingen colleague.
In 2005 they talked at Castel Gandolfo for four hours.
But as Küng brings out clearly in this book, their theological methods were almost at opposite poles.
For Küng, the historical-critical exegesis of the Bible is the measure of all things. He sets out in this way to reach the "Jesus of history" who is in his person the Christian message. This approach became central to Küng's systematic theology - the bar of judgement before which all positions stand or fall.
But there are many incompatible versions of the "Jesus of history" on offer, and Joseph Ratzinger, for his part, applied the same historical-critical method only within limits and with restraint, subordinating it to dogma and tradition.
For him the marriage of the Gospel message with Hellenistic philosophy was providential, and cannot be discarded.
Rather than a doctrine of Christ "from above", however, Küng presents one "from below" - from where people are. As he does not fail to record, his books have been best-sellers and his lectures have been crowded out. This is because he is dealing head-on with issues that people want to hear about in terms they understand.
Reflecting in this book on the struggle, he accepts that at the level of power, the others may so far have won; but on the parish level "the opposition has by far the largest majority". He draws a comparison between the "glittering façade of papal demonstrations" and the "lamentable state of Church and clergy" behind it.
Locally, "millions of people are running away". The "justified hopes and expectations" raised by Vatican II "have not been fulfilled". The Church had all the tools of renewal to hand, only to shrink back.
The reforms "were stifled both in the public arena and behind the scenes". He draws attention to the downgrading of the council's doctrine of collegiality - that the Church is governed by the college of bishops, with and under the Pope - and focuses on John Paul II's reconstruction of "an imperial papacy".
The bishops had walk-on parts. Küng cannot think of a single decisive reform emanating from the bishops' synod. Can anyone?
Paul VI, who had seen the collegiality doctrine through the council, himself began the process of attenuating it, above all when he issued his 1968 encyclical letter, Humanae Vitae, reaffirming the traditional ban on contraception. This non-collegial act sparked off a crisis in the Church.
The huge debate quickly shifted from the question of birth control to the question of authority.
The Pope had set aside the advice of his commission because he felt unable to go against the teaching of his predecessors. His spokesman at the time made it crystal clear that the encyclical had no claim to infallibility, but Küng maintains the opposite; he argues that it was in line with the constant teaching of the world's bishops up to that time, and they also are considered infallible when in such unison.
The vast majority of theologians do not agree with Küng, but believe the encyclical can be revised.
There is no sign of that happening, however, and certainly the influential conservative ethicist Germain Grisez in the United States is one who has argued for its infallibility precisely on the ground of its congruence with constant episcopal teaching.
The showdown came when Küng published his book Infallible?, proposing the substitution of a claim to "indefectibility". There is nothing in the Bible, Küng notes, to suggest that Peter had any charism of infallibility, and plenty of evidence to the contrary. But he had now opposed a defined doctrine.
He discussed his position intensively with his peers but it is not clear how far he ever really took into account the criticisms they made. Karl Rahner as a dogmatic theologian voiced an anguished protest. "Your book", he told Küng, "is a deadly threat to my Catholic faith."
Küng was in the firing line as never before. There is room in the Church for only one Pope, and the massed guns overwhelmed the Swiss theologian. His account is very detailed, drawing on inside knowledge and documentation. The passage where he recalls his mood and behaviour in the subsequent "fearful" months - the worst of his life - is almost too painful to read.
Clearly he came to the very edge of physical, mental and psychological breakdown. "Church of God, my mother", asked Yves Congar, arguably the greatest theologian of Vatican II, "what are you doing with this difficult child, my brother?" Despite losing his licence to teach as a Catholic theologian, Küng remained steadfast, and to this day is a priest in good standing.
His opponents expected that without his licence, he would be finished. He has proved them wrong. He is a prophet on fire with the message of Jesus Christ and driven to communicate it to everyone who will listen. Despite my deep reservations, I can say that when I have interviewed him in Tübingen or heard him speak in London, I have gone out believing in God more that I did when I came in.
John Bowden's translation from the German is superb. As is almost inevitable in a book of this size, however, there are some erroneous references and a scattering of literals; and the interesting photographs are not deployed in three sections, as announced in the index, but all appear in one chunk.
Küng promises a third and final volume, if he lives to write it.
In his ninth decade, as he says, that is "in the hands of Another".
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