The Church of Rome is the world's oldest surviving international organisation, the Vatican the world's smallest sovereign state.
By David Willey
BBC Rome correspondent
The Pope, ruler of this rather anomalous political entity, elected for life by a small body of cardinals, is also the spiritual leader of a worldwide community of more than a billion believers.
The Pope has travelled far and wide during his 25 years at the helm
A tiny Roman bureaucracy at the hub of the Church struggles to keep them in theological harmony and order.
Not an easy task in a world that's increasingly abandoning the religious for the secular, and where religious pluralism is the buzzword.
Kaleidoscope of memories
For 25 years I have followed John Paul II on his travels around the world on an average of four journeys every single year, except during the period when he was hospitalised and convalescing after the attempt on his life.
Pilgrimages he calls them, pastoral visits to encourage his flock in their religious faith, and create new saints and blesseds as local role models for Catholics to follow.
He has travelled to more than 100 countries, and met their leaders.
Only two major countries have failed to issue an invitation, Russia where a deeply suspicious Orthodox Church still vetoes a Papal visit, and China, which set up an independent so-called "patriotic" Catholic Church which does not answer to Rome.
My personal kaleidoscope of memories of these trips is still vivid.
I watched the Pope make his first ever visit to a mosque in Damascus, removing his slippers just like any other Muslim worshipper.
I recall flying with him across the North Pole, crammed inside a Japanese airliner, standing by his side in a tropical rainforest in Togo while African Animists prayed to their deities.
I have seen him walk down the ancient nave of Canterbury Cathedral in England side-by-side with the head of the Anglican Communion, laughing with a group of toddlers on the stage of a martial arts centre in Tokyo under a huge banner reading "Pope is Hope" and inspiring a vast crowd in a Warsaw football stadium during the dark days of martial law in Poland.
I remember him paying his respects at the tomb of Mahatma Gandhi in Delhi and tapping his feet to the rhythm of barefoot African dancers during one of his colourful and vibrant tours of Southern Africa.
But tourism it is not.
I remember the Pope berating a couple of Australian journalists he invited to join him in the front of his plane who rather irreverently asked him if he enjoyed looking out of the window to pass the time flying across the seemingly endless desert of their continent.
No, he said, I don't do tourism, I read and I meditate.
During the years before his election as Pope when he was bishop of Krakow under the Communist regime in Poland, he installed a special writing desk and reading light in the back of his car in order not to lose precious time while being driven from one part of his diocese to another.
During the years of international travel innocence before the terrorist attacks of 11 September, 2001, security was strict but not draconian.
Frailty has made the logistics of papal visits more difficult
A couple of Swiss Guards in mufti and the head of Vatican security still travel on the plane, but the cosy informality of the early papal travels can no longer be allowed.
The press in the back of the plane and members of the papal entourage no longer stay in the same hotels.
The Pope, who once materialised in front of me as I was having my breakfast flying across the Pacific Ocean - I thought he was an apparition - is now too incapacitated to circulate among those travelling on his chartered jetliner.
The wheelchair period of the papacy has begun.
Here in Slovakia, where the Pope is on his 102nd foreign trip, you begin to understand the logistical problems of protecting and transporting around a frail old man who simply refuses to have his life's mission compromised by the infirmities of old age.
The aspect of papal travel that has most impressed me is the diversity in the ways the Catholic religion is lived in different parts of the world.
While Rome struggles to impose uniformity, in practice, the Catholic Church seems to me to have invented a surprising diversity in its interpretation of the Christian message.
I often wonder if the Pope himself has noticed on his travels how rich and varied is the fabric which the successors of Saint Peter have woven over the centuries.