Visiting Greece, Syria, Ukraine, and Armenia over the next six months, the pope will travel to countries where no modern pontiff has been and where Catholics are a small minority.
His route appears to many observers as an ecumenical obstacle course, taking him deep into the historic territory of Orthodox Christianity, where the centuries-old divisions with Catholicism are still very much alive.
``It's going to be a real test for him, an ecumenical test,'' said U.S. Jesuit Father John Long, who worked on Christian unity issues for many years at the Vatican.
For decades, these countries were considered off-limits for papal travel, in part because of ecumenical tensions. But this time around the pope and his aides have knocked harder on the doors, perhaps aware that the pope's traveling time is not limitless. As a result, he will arrive in Greece and Ukraine without a clear invitation from the major Orthodox churches.
For some experts, that has raised questions about the lasting legacy of these visits.
``I want to put a positive spin on it, but I can't really convince myself. I'm afraid there could be some real difficulties that develop out of this,'' Father Long said.
In Greece, where the pope will travel May 4-5 to commemorate St. Paul's evangelization in Athens nearly 2,000 years ago, the Holy Synod of the Orthodox Church of Greece voted not to formally oppose the visit, stressing that it was a personal pilgrimage by the pope.
But some Orthodox clergy and faithful in Greece remain alarmed all the same. One conservative union of Orthodox priests described the pontiff as an ``arch-heretic'' and ``the two-horned grotesque monster of Rome'' and predicted massive protests during the visit.
The depth of such hostility is surprising to most Western Christians. Father Long said it reflects not only age-old animosities dating to the Catholic-Orthodox schism of 1054, but more recent misgivings about the ``Westernizing'' of Greece, too.
In Ukraine, the situation is complicated by the local split in the Orthodox community. Two smaller Orthodox churches are supporting the papal visit, but the majority Ukrainian Orthodox Church--which is linked to Moscow--has bitterly opposed the visit. Its officials have also warned of protests and demonstrations when the pope visits the cities of Kiev and Lviv in late June.
The prospect of creating new barriers with the Russian Orthodox Church is something ecumenists do not relish. Some believe the pope's advisers, weary of Russian Orthodox Patriarch Alexei II's rebuffs of papal overtures, have adopted a ``take it or leave it'' attitude toward the Moscow church when it comes to papal travels.
Father Long said the pope quite rightly wants to reward the small Catholic community in Ukraine for witnessing the faith during decades of communist persecution. But given the complex set of divisions and tensions with the Orthodox in Ukraine, ``if he tries to reach out beyond the context of Catholics...he's in trouble,'' Father Long said.
One sensitive point is the provisional plan that calls for the pope to celebrate a Ukrainian Divine Liturgy in Kiev, an area in the eastern part of the country where the heaviest concentrations of Orthodox live. Many Orthodox view the Catholic Ukrainian rite as an appropriation of their own liturgy, and to see the pope presiding over such a celebration in Kiev would be like a ``slap in the face'' for them, Father Long said.
The papal visit to Armenia, planned for September, will be easier. The Armenian Apostolic Church is one of six independent Oriental Orthodox Churches that split with Catholicism in the fifth century over a council's explanation of the human and divine nature of Christ.
Those sentiments have died down, and all signs are that the pope will be welcomed warmly by the current Armenian patriarch, Karekin II, when the two lead celebrations to mark 1,700 years of Christianity in the Caucasus nation.
Syria, where the pope will spend three days on his St. Paul pilgrimage in May, is an ecumenical oasis on his itinerary. The two main Orthodox communities in Syria have not only invited the pope but are on the planning committee for the visit, Armenian Catholic Bishop Boutros Marayati of Aleppo, Syria, said in a telephone interview.
``Here the Orthodox are in an Arab country, and there's a very different mentality than in Greece or Russia. All the Christians here feel united because we are witnessing Christianity as a minority in a Muslim majority. Our relations with the Orthodox are excellent,'' he said.
The pope has long desired to walk these paths in the East. As he finally sets out this year, he knows he will be moving across an ecumenical terrain that is mostly uphill, and where missteps are easily made.