The papacy is one of precious few institutions in the modern world that does not change very much. It maintains the appearance and traditions of the Church much as it has done since the later days of the Roman Empire. One of those traditions has been that the Pope would serve until death.
Catholics were brought up to believe that the election of a Pope, by a college of cardinals in solemn and prayerful conclave, was done with divine guidance and would end when God willed.
No wonder then the news early yesterday stunned the world. Resignation was thought to be no more an option for an ailing Pope than for Britain's monarch, possibly even less likely since it is a mere 77 years since a royal abdication. Historians had to delve back 600 years, to the Middle Ages, for a papal precedent yesterday.
Pope Benedict's decision has taken his Church by surprise. At 85, he seems no more frail than many ageing pontiffs have been. There were signs of health problems but nothing out of the ordinary for someone of his age and no conjecture about his continued ability to perform the functions of the office.
The Vatican had clearly not had time to alert its hierarchy around the world and New Zealand's bishops were as surprised as anybody else. They accepted and respected the Pope's explanation that he no longer felt his strength was up to the task. They called his decision courageous and undoubtedly it is. He knew the tradition he was breaking and decided it was time to break it.
No good is done by those who continue in high office when they know their faculties are failing. It takes great honour and often personal sacrifice to step down when the decision is entirely yours and you alone know your condition.
Pope Benedict has set a powerful new precedent for the modern papacy. Every Pope from now on will face the question whether he should continue in old age. The first consequence of Benedict's retirement at the end of the month is likely to be the election of a successor much younger than he was when chosen eight years ago.
The election of the German Cardinal Joseph Ratzinger dismayed many Catholics, not on account of his age but his conservatism. He had been an author and leading enforcer of the unbending moral and doctrinal positions of John Paul II. After weathering the strictures of John Paul for so long, Church liberals, especially women, had hoped for a moderniser.
Pope Benedict has been better than they feared. His utterances have been gentle, thoughtful and highly intelligent, reflecting his grounding as a scholar and theologian. He has been able to engage those who disagree with him and realised early in his papacy that he was a voice of Christianity at a delicate moment in history.
In an early speech at his old university in Regensberg he quoted a Byzantine emperor's view that Islam was inherently violent and apologised after an outcry from Muslim clerics. A few months later he toured Turkey and prayed in a mosque.
But he was to face a far bigger challenge from the disclosures of priestly paedophilia that erupted nearly everywhere. The extent of offending has surprised Catholics and their clergy as much as anybody. Priests had been shielded by bishops who believed in the sanctity of the confessional, the goodness of mercy and repentance. The Pope stands at the head of a Church accused of a systematic cover-up.
But more than any of this Benedict XVI will be remembered for his decision to retire. It is ground-breaking and refreshing. It places high hope in a new Pope.