The appearance is frail, the shoulders are stooped and about the face there has long been that ashen quality of those who know they are touching the outer limits of mortality.

But even these signals did not prepare the world's 1.2 billion Catholics for the announcement that the 264th successor to the apostle Peter would resign his pontificate at the end of the month.

For hundreds of years it has been one of the verities of Catholicism that retirement is no option for the man elected to wear the onerous white skullcap and red shoes of the papacy.

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Yet in reaching his decision, unprecedented since the Middle Ages, Pope Benedict XVI has confounded those who regard his ministry as retrograde, a reign of uncompromising doctrinal sternness that has caused millions of liberal Catholics either to disobey the Church's social teaching or leave the religion altogether.

There is nothing orthodox about yesterday's announcement. Indeed, it might well be seen as the most radical act of any pope for centuries.

Storms of controversy were already battering the Vatican when in April 2005 the Bavarian theologian Joseph Ratzinger was elevated to the papacy at the age of 78.

And it was a sign of the harm caused by appallingly errant priests and nuns – not only to their sex-abused victims but to Catholicism as a whole – that never before had a pope been so globally vilified as this one.

Contrary to the prevalent view at the time, Benedict's defenders maintained that during the long, charismatic pontificate of his predecessor John Paul, Cardinal Ratzinger was the one who repeatedly urged the Church to confront "the filth marring the priesthood".

The controversies haven't ceased. While Benedict's strong condemnation of greed, banking fraud, world poverty and war have shown him to be on the side of the angels, his opposition to euthanasia, homosexuality, same-sex marriage and women priests remains as implacable as ever.

Last year the Holy See railed against the influence of American nuns in particular, accusing them of detrimental "feminism" for upholding such views in the name of social justice.

Defenders of the sisters replied that it was far too late for Vatican censure because, for centuries, women had gathered in religious orders as an escape from patriarchal discrimination, and they certainly weren't going to be silenced now.

And yet, almost unnoticed, there has been a shift in Benedict's teaching against artificial birth control. In 2010 during an interview in his native Germany, the Pope said that, in the case of prostitution, condom use could "perhaps" be justified to help prevent the spread of Aids. His remarks, in the context of Africa, were hedged with conditionals.

Even so, many Catholic theologians detected that this could be a big moment with far- reaching consequences. Was he preparing a softening of the line for his successor?

It was also two years ago in Germany that the Pope hinted he would not wish to continue in the papacy if his health were failing.

The telling phrase in yesterday's statement was that he no longer had the physical or mental strength to carry on with his job. Having taken the decision Benedict was "very serene", the great burden of office about to be lifted from his shoulders.

All this, of course, is in marked contrast to the final days of Pope John Paul, whose death-bed sufferings became a world drama in themselves and a not unfitting end for someone whose papacy had been characterised by extrovert spirituality.

Benedict is different. His aura of scholarship initially made him seem austere. Yet those who know him describe him as a shy, contemplative, softly spoken man of exquisite manners, qualities that were movingly on show during the memorable state visit to Britain which began in Scotland on September 15, 2010.

The day had begun tentatively with little sign of the anticipated thousands lining Princes Street for the St Ninian's parade. But as word spread of the airport arrival of the Pope's chartered Alitalia jet, Shepherd One, the first multitude of the day gathered and, as the sun shone, Edinburgh, to the sound of a thousand pipers, looked magnificent.

By the afternoon it was Glasgow's turn, with close on 70,000 "pilgrims" transforming Bellahouston Park into a vast, open-air cathedral to hear Benedict celebrate Mass in Latin, the language he chose, also, for yesterday's announcement.

Its revelation came on the feast day of the Pope's chosen-name saint, Benedict. On February 28, he will leave his Vatican chambers on the feast day of Saint Hilary, who died in 468 after seven years as pontiff. Just one year less than the length of Joseph Ratzinger's beleaguered reign.