THE Pope's resignation is a potential watershed for the Catholic Church, the most courageous and definitive action of his career.
When he steps down at 8pm on February 28, Benedict will be the first Pope to resign in 598 years - if you count Gregory XII in 1415, who was pushed - or 719 years if you go back to Celestine V, who resigned in 1294.
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Shocked faithful gather in St Peter's Square
News of the resignation of Pope Benedict XVI came as a complete surprise, reports European correspondent Nick Miller in St Peter's Square.
It has been part of the papal self-understanding for centuries that ageing, frailty and suffering are part of the burden, and to resign is a betrayal. Pope Benedict, Pope John Paul II's right-hand man, watched his predecessor's long decline and the paralysis it caused in the Vatican - the intrigues, infighting and agendas.
Elected Pope John Paul's successor in 2005, when he was 78, Pope Benedict has clearly decided not to condemn the church to the same hiatus. He has decided, after serious examination of his conscience, that the challenges of the modern world require a man who is mentally and bodily strong, and that at 85 he no longer is.
The announcement, in Latin, at a meeting of cardinals on Monday, came as an utter surprise to the church, but with hindsight it should not have. He had previously argued that under some circumstances it would be his duty to resign, and he twice prayed at the tomb of Celestine V, which could be seen as a symbolic gesture of intention.
His resignation is important not only for what it means to the church in 2013 but as a precedent for future popes, enabling the church to be regularly reinvigorated. And it needs invigoration.
Pope Benedict XVI leaves a church facing the same challenges it faced when he was elected. Some issues, such as the ever-expanding clergy abuse crisis, have been considerably exacerbated.
Debate about his legacy has begun, but it surely contains few heroics to match his departure. He was not a global political figure like Pope John Paul II and his priorities were more the internal life of the church, where he was a respected if very conservative theologian and teacher. A piano-playing introvert, he has been described as intellectually remorseless but personally timid.
Electrifying reaction to Pope's shock exit
Social media is bursting at the seams with discussion, speculation and a healthy dose of humour over the Pope's surprise resignation.
Before the 2005 conclave of cardinals that elected Pope Benedict, the main challenges facing the church in the West included an increasingly secular and post-Christian Europe, collapsing faith among the young, sexual morality, bioethics, and collegiality (a code for curbing the power of Vatican bureaucrats and giving local bishops more autonomy). For Africa and Asia, relations with Islam were vital, and in Latin America poverty and relationships with Protestants were issues.
Seven years on, all remain just as challenging, and clergy sexual abuse (including the church's response) has moved right up the agenda.
Pope Benedict's teaching has been hailed, especially his first encyclical, Deus Caritas Est (God is Love), and he drew huge crowds when he travelled, but his tenure was racked by controversy - often self-inflicted.
Quoting a dismissive remark about Islam by a Byzantine emperor in a speech at Regensburg in 2008 led to riots across the Islamic world, including church-burnings and deaths.
Other incidents of foot-in-mouth disease included suggesting on a trip to Africa that condoms made AIDS worse, reconciling temporarily with the ultra-traditionalist Society of St Pius X until it emerged that a key bishop was a Holocaust denier, downplaying the negative effects of colonisation on a visit to Latin America, and offending Jews by reviving a Mass that prayed for their conversion.
Last year's Vatileaks scandal, for which his butler was convicted of stealing and leaking documents, showed a church crippled by factions and infighting, and demonstrated how he had failed to curb the Curia - the Vatican bureaucracy that he had been expected to reform; he seemed to have lost control of the intrigues swirling beneath him.
But the issue that showed his tendency to vacillate was clergy sex abuse, including allegations that he was soft on a molester when he was Archbishop of Munich. He understood the crisis far better than John Paul II and did act to tackle it, but inconsistently, and with none of the rigour with which he pursued potential theological dissidents such as the former Toowoomba bishop Bill Morris.
Time will lend a fuller perspective, but the Pope preferred manning the battlements to sorties beyond the walls. If his pontificate was often ineffectual, his abdication may prove the opposite.