Pope Benedict XVI has made a spiritual journey in his life from liberal young priest to conservative pontiff, but the BBC's Edward Stourton says that his decision to resign reveals traces of the radical he once was.
Pope Benedict is a conservative in the deepest sense of the word. He believes that tradition reflects important truths and should be respected. That is why the announcement of his resignation, a radical decision which overturns centuries of papal tradition, has sent such shock waves through the Catholic world.
But Joseph Ratzinger - as he was known before his election - has lived a life of surprises.
One of the most recent was the way he managed his visit to Britain in 2010. In the months leading up to his arrival, the British press predicted it would be a disaster, and that he would come to lecture and not to listen.
In fact he won widespread admiration for his humility and warmth, and for the intellectual vision he showed when he addressed members of both Houses of Parliament in Westminster Central Hall.
But the most important surprise of his career was his transformation from revolutionary to conservative in the 1960s.
In 1962 Pope John XXIII summoned the bishops of the world to the Second Vatican Council. He wanted to open up the Roman Catholic Church to the 20th Century, and the Council introduced a wide-ranging set of reforms. It replaced the Latin mass with worship in local languages, emphasised the Church's duty towards the poor, and, for the first time, the Church recognised a universal right to religious freedom.
St Peter's Basilica was transformed into a makeshift parliament for the Council debates, which were conducted in Latin. Joseph Ratzinger was there as a "peritus", or theological adviser, working for Cardinal Frings, the Archbishop of Cologne and one of the leading modernisers.
With a reputation even then for formidable intelligence, the young Father Ratzinger helped draft some of the key speeches with which Cardinal Frings swung the meeting behind the reformers.
In an interview many years later, the future Pope said that he had then believed that the old theology "had to get out of its armour. It also had to face the situation of the present in a new language, in a new openness".
A greater freedom, he argued, also had to arise within the Church.
In 1966, just after the end of the Council, Joseph Ratzinger took up a new academic position at Tubingen University, which was at the time the flagship of liberal theology in Germany. So when student revolutions convulsed Western Europe and the United States two years later, he was a first-hand witness.
The experience changed his life. Years afterwards he recalled the "traumatic memory" of a group of theology students at Tubingen who put out a flier claiming that "the New Testament is a document of inhumanity, a large scale deception of the masses", and asking "so what is Jesus's Cross but the expression of a sado-masochistic glorification of pain?".
Prof Ratzinger, as a German who had grown up in the 1930s, understood all too well what a totalitarian regime could do. He saw a new left-wing totalitarianism at work in the revolutionary spirit of the 1960s.
"It is true I saw a new spirit creeping in," he said. "A spirit in which fanatical ideologies made use of the spirit of Christianity… Here I saw very clearly… that there was an abuse of the Church and the faith, which were used as instruments of power."
He concluded that a return to traditional teaching was the only rational response.
Joseph Ratzinger's new conservatism earned him a top job in the Vatican. In 1981 Pope John Paul II appointed him Prefect of the Congregation of the Doctrine of the Faith, Rome's theological chief of police. The way he cracked down on theologians who questioned orthodox teaching earned him the nickname "the Panzer Cardinal".
Although many liberal Catholics were impressed by the gentleness of Benedict's style as Pope - another of his surprises - the conservative views he adopted in the aftermath of the 1960s student revolutions had a profound impact on his pontificate.
On most of the sensitive areas of Church teaching - women priests, contraception and homosexuality, for example - he was uncompromising. And he campaigned against what he strikingly called the "dictatorship of relativism" in post-Christian western societies.
But his decision to resign suggests the radical young Fr Joseph Ratzinger is still there in the old Pope Benedict.
His predecessor John Paul II struggled on in office to the bitter end. John Paul showed astonishing courage in the way he strove to keep going in the face of terribly physical suffering, and his willingness to sacrifice himself for the sake of his office has become part of his legend. He was a heroic figure who provided a model for future popes.
Pope Benedict has always lived in his predecessor's shadow, but by resigning he has now offered future Popes a different model.
Even his many enemies have, for the most part, conceded that he is a humble man and that he is motivated above all by a desire to serve the Church.
The message of the decision announced this week is that the Church can sometimes be best served by a recognition of human frailty - which touches everyone, even Popes.