By David Willey
BBC News, Rome
Pope Benedict XVI travels to Turkey on Tuesday for what is arguably the most dangerous, delicate and contested visit outside Italy made by any pope in modern times.
There will be no open-air cruise through Istanbul
Not only is the spiritual head of the world's 1.2 billion Roman Catholics making the first visit of his pontificate to a predominantly Muslim country, but he will also be confronted by hostility from vociferous right-wing Turkish Islamic nationalists, who have already made it clear that he is not a welcome guest.
Benedict is also mindful that Ali Agca, the gunman who tried - and failed - to assassinate his predecessor John Paul II in Rome in 1981, was a Turk.
Agca has written to Benedict from his prison cell (he is now serving a further jail term in Turkey for another unrelated crime) advising the pontiff that he would be wise to postpone or cancel his trip.
In a change of plan, Turkish Prime Minister Recep Tayyip Erdogan has said he will meet the Pope at Ankara airport on Tuesday. Mr Erdogan had earlier planned not to meet the pontiff, saying he would have to travel to a Nato summit in Latvia.
The Pope is aware that he will have to weigh very carefully the impact of every word he pronounces during his four-day stay in Ankara and Istanbul.
A lecture on "Faith and Reason" he gave in September during his last foreign visit - to his native Germany - caused a political furore and widespread offence in the Islamic world.
Benedict quoted a Byzantine emperor, who said that Islam was violent and irrational. Not true, replied his critics, demanding an apology.
Since September the level of invective may have died down, but the Pope's words will not be easily forgotten, despite his claims that he was misunderstood.
There will be no triumphant crowds applauding the Pope in his popemobile on this pilgrimage.
The Pope has faced much criticism in Turkey even before his arrival
In fact there will be no popemobile. He will travel only in motorcades of closed armour-plated limousines protected by a huge security cordon organised by his hosts, the Turkish government.
The Pope's visit to Turkey was originally planned by the Vatican with a view to improving relations with the Orthodox Church.
The spiritual leader of the world's Orthodox Christians, Patriarch Bartholomew, happens to have his headquarters in Istanbul, the now mainly Muslim city which for centuries was known as Constantinople.
An ethnic Greek - but a Turkish citizen - Bartholomew has a minuscule local Christian flock of only about 3,000 people - a mere speck in an ocean of Islamic believers.
The Pope hopes for reconciliation with Patriarch Bartholomew I
But at the same time the Patriarch is also leader of a worldwide group of Christian believers who separated from Rome nearly 1,000 years ago.
The Orthodox are present mainly in Russia, in the Balkans and among a diaspora now scattered around the world.
The meeting between the leaders of the two churches is to be celebrated on 30 November, a feast day of huge importance to the Orthodox - that of their patron, Saint Andrew the Apostle, who brought Christianity to Asia Minor.
It is also of great significance for Pope Benedict. One of the long-term aims of his pontificate, he says, is to try to help heal the wounds of bitter theological disputes which have divided Christendom, some lasting hundreds of years, so that one day the whole Christian family might again be gathered into a united body.
There are therefore three quite separate aspects to the Pope's visit to Turkey: his official visit to the secular Turkish state; his ongoing unofficial, at times acrimonious, dialogue with the world of Islam; and his formal visit to the headquarters of the Orthodox Church.
When I accompanied Benedict's predecessor John Paul II to Istanbul at the start of his pontificate just over a quarter of a century ago, John Paul did not once utter the word "Islam" in any of his speeches.
Benedict will become only the second pontiff to visit a mosque
But the world has changed a lot since 1979.
The legacy of the defunct Ottoman Empire has receded even further into history; Turkey is knocking at the door of the European Union; Europe is becoming ever more secular; the Christian presence is haemorrhaging away in the war-torn Middle East; and the Roman Catholic Church is now competing vigorously with Islam for converts in sub-Saharan Africa.
In Istanbul, in a last-minute addition to his programme, the Pope will be visiting the city's famous Blue Mosque. It will be only the second time in history that a pope from Rome has entered a Muslim place of worship.
His whole visit will be heavy with religious symbolism. He will also enter Haghia Sophia - which was for 1,000 years the largest church in Christendom, was then converted into a mosque after the fall of Constantinople, and is now a secular building, a museum.
Every phrase the Pope utters during his Turkish visit will be picked over for the light it sheds on his vision of how the separated Churches of Christendom and the Muslim world may coexist and even co-operate in the fast-changing and interconnected world of the 21st Century.