John Charles McQuaid, the deeply conservative Archbishop of Dublin, returned from Rome in 1965, declaring that the Second Vatican Council would have no impact on his flock. "No change will worry the tranquillity of your Christian lives,'' he told them.
He could not have been more wrong. The council's reforms – the so-called 'aggiornamento' – were radical and fundamental, sweeping away the Latin Mass, breaking down barriers between clergy and laity and opening the door for ecumenism. They would shake McQuaid's world to its foundations.
But the archbishop's confusion was understandable, for this revolution had been led by the most unlikely of revolutionaries.
Cardinal Angelo Giuseppe Roncali had been known as a good and gentle pastor, with a distinguished record of rescuing Jews from Nazi persecution.
He was 77 when he became Pope John XXIII and there was not the slightest reason to believe that he would be other than a caretaker pontiff.
He did not seek, or expect, office. Indeed, he famously came to the conclave that elected him with a return rail ticket in his pocket.
But Roncali had served his Church for many years and had watched it grow stale, tired and out of touch. He knew what it needed and, where a shrewder politician might have negotiated and compromised, he simply set about doing it.
John XXIII is an example of a heartening aspect of structured society; proof that in even the most conservative organisations there are radicals with a willingness to tackle fundamental change.
Like John, they may not seek the highest office, or even particularly want it. But if it falls to them, they will not waste the opportunity.
Lyndon Baines Johnson was a good ol' boy from Texas, selected for vice-president to give conservative balance to the radical stance of President John F Kennedy.
He was part of the white southern establishment and, although he had never expressed any animosity towards racial integration, or improved civil rights for blacks, neither had he shown support for these developments.
But when Kennedy's assassination landed him in the White House, Johnson became a powerful advocate of the civil rights cause. Where Kennedy had struggled to pass a civil rights bill, Johnson succeeded.
He also passed the Voting Rights Act and the Fair Housing Act. He used his experience as Senate majority leader, along with a legendary Texan toughness, to push through this highly controversial legislation – twisting arms and cutting deals in a way Kennedy had always found difficult. Mikhail Gorbachev proved an even greater surprise. He was a Soviet career communist, who had worked his way up through the party, from university branch, to youth wing, to the politburo.
He had impressed as a fine intellect and good organiser, but had never taken the dangerous step of opposing party orthodoxy.
Yet, just as John XXIII had seen the Roman Catholic Church grow stale and out of touch, so Gorbachev had watched the Soviet empire turn to a tottering wreck under the tyrannical leadership of tired old men.
When that leadership fell to him, he swept it all away; the power of the party, even the Soviet Union itself. He cleaned out the Kremlin, brought down the Berlin Wall and lifted the Iron Curtain. Little thanks he got for it, but he brought us peace in his time.
Closer to home, and on a smaller scale, the same might be said of Terence O'Neill and Sean Lemass. An odd couple. O'Neill was an ascendancy unionist, with a background of Eton, Sandhurst and the Irish Guards. Lemass was an ex-IRA man who had fought in the 1916 Rising.
They had both achieved office after the retirement of long-serving and deeply entrenched politicians – Basil Brooke in Northern Ireland and Eamon de Valera in the Republic. There was no reason to suppose that either man would step far outside the groove worn by their predecessors.
But both realised that the policy of shaking fists across the border could not continue. Their meetings in 1965 began a process which changed utterly the relationship between the two parts of this island.
O'Neill failed in his attempt at reform within Northern Ireland, but the friendly cooperation we see today between Dublin, Belfast and London owes much to this unlikely revolutionary.
The focus of leadership has once again switched to the Vatican, where the conclave of cardinals must elect a successor to Pope Benedict XVI.
The new pontiff will lead a Church beset by problems. It is mired in the worldwide scandal over sex-abuse by priests, which many of the cardinal-electors are blamed for covering up.
It is trying to recover from the 'VatiLeaks' scandal, which pointed at corruption in Rome, including the blackmailing of homosexual clergy.
It is struggling to come to grips with wider issues, such as the ordination of women and the status of gay couples.
The Church seems as out of touch as it was when Cardinal Roncali stepped into the Shoes of the Fisherman.
Is there, in the ranks of the ageing cardinals, another revolutionary capable of turning it all around?
Observers say it is unlikely. History says otherwise.