The papacy of John Paul II has been one of the most remarkable in the history of the Catholic Church.
Only two other popes - three if you include St Peter - have reigned longer than the present pontiff, who marks his 25th anniversary on Thursday.
Few popes of any century have had such an impact, either on the Church, or on the times in which they have lived.
John Paul II really has become the "universal pastor", using air travel and the mass media to take his message out to the world.
It has become a familiar sight - the Pope arriving in a foreign land, kissing the ground, and then preaching at an open air mass to perhaps a million people.
"Through these amazing journeys he has shown the Catholic Church to the world as never before," says John Wilkins, editor of The Tablet.
In 25 years, John Paul II has been to almost every corner of the world, re-defining the papacy for a modern age.
Whoever succeeds him will feel obliged to follow his example. Media skills and fluency in several languages has now become a requirement for the job.
"For millions of Catholics, John Paul II has been a father figure," says Madeleine Bunting, a writer on church affairs.
"We have never had such a well-known pope, and such a popular pope."
Yet when he was elected in 1978, few outside Poland had heard of Karol Wojtyla, then the archbishop of Krakow.
The cardinals who chose him knew they could count on him to uphold traditional beliefs at a time when many Catholics were questioning the teachings of the Church.
During his papacy, there has been no wavering in the Vatican's position on contentious social issues such as birth control, abortion and divorce.
The Pope played a key role in the downfall of Communism in Eastern Europe
Neither has the celibacy of the priesthood or the role of women in the church ever come up for discussion.
This, too, is part of his legacy. In fact, the uncompromising views of John Paul II may now limit the room his successor has for manoeuvre.
"The next pope is going to have a very difficult time untying things like birth control, abortion and women priests," says Madeleine Bunting.
"With all of these sensitive issues, John Paul II has made it very, very difficult for his successor, because a pope cannot undo the teachings of his predecessor.
"For example, he has been so categorical that women priests are not acceptable that it will take a long time - decades - for that to be changed gradually."
End of Communism
But while John Paul II has been a conservative in terms of doctrine, he has tried to promote social justice, not least in his homeland.
As a young man growing up in Poland, Karol Wojtyla witnessed the rise of Nazi Germany. Then after World War Two, he faced the challenge of being a priest in a Communist state.
"When the church elected a Polish pope in the middle of the Cold War, everybody felt it was a real political statement," recalls Father Thomas Reese, an authority on the workings of the Vatican.
A major theme of John Paul II's papacy has been his attempt to reach out to other faiths
"John Paul II played an extremely important role in bringing down Communism in Eastern Europe. His election was inspirational."
The proactive style of John Paul II has underlined the fact that a pope is not just a spiritual leader; he is also a player on the world's diplomatic stage.
He spoke out against the war against Iraq, trying in vain to persuade the United States and Britain to hold back from military action.
A major theme of his papacy has been his attempt to reach out to other faiths, in search of reconciliation after centuries of hostility and suspicion.
He has travelled to Islamic countries, becoming the first pope to set foot in a mosque. As a gesture of religious tolerance, it took on new meaning after the events of 11 September 2001.
'John Paul the Great'
Marco Politi, one of Italy's best-known Vatican watchers, believes John Paul II will be remembered not only as a great pope, but also as a leading figure of the 20th Century.
"Twenty-five years ago, the pope was an important personality for Catholics, maybe for Christians," he says.
"Now he is a spiritual leader accepted and recognised by people of different faiths."
But while some in the Church are already talking about "John Paul the Great", others are more doubtful about his legacy.
They say that during his reign, the Vatican has exercised too much power, and is now less tolerant of dissent.
Some want to see a different kind of papacy, with bishops around the world having a greater say in how the Church is run. That would be a challenge to the authority of the Church's central bureaucracy, the Curia.
John Wilkins, editor of The Tablet, believes that after John Paul II there may be a change in the relationship between the Pope and the Church outside Rome.
"It is a curious paradox of a very centralised papacy that in some ways the Church has never been so open in the direction that it follows now," he says.
"John Paul II has laid the foundations for the future. He is the last pope of the 20th Century, rather than the first one of the 21st Century."
But whoever eventually succeeds him, he will be a tough act to follow.
Father Thomas Reese believes the Pole brought unique skills to the papacy, and whoever succeeds him will not be a carbon copy.
"There is no-one like John Paul II," he says with a smile.
"We would have to clone him, and the Church is against cloning."