By Jan Repa
BBC Europe analyst
The Pope's meeting with Patriarch Bartholomew will be keenly watched
Pope Benedict XVI, spiritual leader of the world's Catholics, is to meet Patriarch Bartholomew - "first among equals" of the leaders of the Orthodox Christian churches - in the Turkish city of Istanbul.
The Pope's visit to overwhelmingly Muslim Turkey has already provoked controversy - with some nationalist and Islamist groups insisting he is not welcome.
However, the Catholic-Orthodox relationship has also been fraught with difficulty, even before the two churches split nearly 1,000 years ago.
On the same day as he meets Patriarch Bartholomew, Benedict XVI will also visit one of the world's architectural marvels.
Built nearly 1,500 years ago by the East Roman (Byzantine) Emperor Justinian, it was known as the Hagia Sofia - or Church of Holy Wisdom.
A lost symbol
Converted into a mosque by the conquering Turks in 1453, it became a museum in the 1920s.
For many Orthodox Christians, it remains the lost symbol of their faith.
Some Muslim groups would like it to be a mosque once more.
If Benedict XVI offers a prayer here, the result could be religious dynamite.
Pope Benedict's visit to the Haghia Sophia will be closely watched
The history of Istanbul - once known as Constantinople - exemplifies the clash of religions, politics and brute power.
Catholicism and Orthodoxy were once twin aspects of the same officially approved version of Christianity, established under the Roman Empire after its conversion in the 4th Century.
Catholicism was dominant in the "Latin" West; Orthodoxy in the Greek-speaking East.
Over the centuries, political, cultural and theological differences widened to the point where the two Churches formally split in 1054.
In 1204, Catholic Crusaders sacked Constantinople.
Though roundly condemned by the Pope of the day, the sack is still seen by many Orthodox as an act of "Latin treachery" - and continues to mobilise anti-Catholic sentiment in traditionally Orthodox countries like Greece and Russia.
It took until 1964 for a Pope, Paul VI, to meet an Orthodox Patriarch of Constantinople, Athenagoras, on neutral ground, in Jerusalem.
Recent Popes and Patriarchs have pledged to work for reconciliation and greater unity.
But significant obstacles remain.
One is the status of the Pope - seen by Catholics as the final arbiter of theological and moral truth.
For the Orthodox churches, such authority derives from the first Seven Councils of the Church - the last of which occurred in 787 AD - whose rulings cannot be altered or added to.
Other differences concern issues like the nature of Holy Trinity; the relationship between science and Faith; whether God can ever be fully understood; or the existence - or otherwise - of Purgatory.
There are also tensions between the various Orthodox churches - with some, like the Russian Orthodox Church, traditionally vying for the "number one" position; and some suggesting that the Patriarch of Constantinople may be too keen on his links with Rome.
One subject which may well come up during Benedict XVI's trip to Turkey is the allegation that Christians are not treated fairly.
In the 1920s, when the Turkish Republic was established on the ruins of the Ottoman Empire, there were 200,000 Orthodox ethnic Greeks in Istanbul.
Today there are 5000.
Istanbul's Orthodox Christian school of theology was closed by the authorities in 1971 - and remains so, despite appeals from the European Union.
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