On the six-day trip, the 94th that he has made outside Italy in the almost 23 years of his papacy, John Paul will visit the Ukrainian capital of Kiev and the city of Lviv in Western Ukraine, the only diocese in the world led by two cardinals.
Reports from Kiev said that demonstrators, including thousands of Orthodox priests and nuns, marched on the residence of the papal envoy on Thursday (June 21) in the latest and largest of a series of protests against the pope's visit.
Elderly women waved icons and signs reading, "Orthodox or death."
The scene was reminiscent of the angry demonstrations staged by Greek Orthodox militants before John Paul's arrival in Athens last month.
In Athens, however, the Roman Catholic pontiff went on to win applause from the Greek Orthodox Synod with his surprise apology "for the occasions past and present, when sons and daughters of the Catholic church have sinned by action or omission against their Orthodox brothers and sisters."
In Ukraine, he will face a far more complicated situation involving both centuries-old animosities between Catholics and Orthodox and bitter, new disputes that arose with the fall of communism a decade ago.
"I am about to go to Ukraine with great hope," John Paul told pilgrims attending his weekly general audience Wednesday. "My purpose is to confirm our brothers and sisters of the Catholic community in the faith and also to promote our commitment to ecumenism."
Opposition to the papal visit comes mainly from the largest Orthodox jurisdiction, the Ukrainian Orthodox Church, which has maintained its links with the Patriarchate of Moscow and announced it will boycott a meeting with the pope on Sunday.
Russian Patriarch Alexii has vetoed a papal visit to Moscow, and both Moscow and the Ukrainian Orthodox Church tried repeatedly to convince the pope to cancel the trip to Ukraine.
The Ukrainian Orthodox Church-Kiev Patriarchate and the Ukrainian Autocephalous Orthodox Church have agreed to send representatives to meet with John Paul in Kiev's Philharmonic Hall.
Kiev is of great importance to both the Ukrainian and Russian Orthodox churches, which trace their origins to 988 when Prince Vladimir and his followers were baptized in the waters of the Dnieper River, which runs through Kiev.
This "baptism of the Rus" took place just 66 years before the Great Schism of 1054 divided Eastern and Western churches.
In 1596, what is now the Ukrainian Greek Catholic Church returned to communion with Rome with the Union of Brest while keeping the Byzantine (liturgical) Rite.
Labeled "uniates" in disparagement by the Orthodox, they suffered persecution over the centuries by Cossacks, czars and communists. Soviet dictator Joseph Stalin in 1945 and 1946 sent the church underground by arresting its bishops, confiscating its property and giving members the choice of joining the Orthodox Church or going to prison.
With the fall of communism the Ukrainian Greek Catholics began practicing their faith publicly again and demanded the return of their confiscated property. The Russian Orthodox Patriarchate protested what it claimed were violent property seizures by the Greek Catholics and accused them of seeking converts among the Orthodox.
A dispute over the "uniates" broke up the last meeting of the Joint International Commission for Theological Dialogue between the Orthodox Church and the Roman Catholic Church held in Emmitsburg, Md., earlier this year.
The Latin Rite Ukrainian Roman Catholic Church, which has historical ties with Poland, was created in 1375 and given Lviv as its seat by Pope Leone XI in 1412. It also underwent persecutions during communist rule.
At the consistory he held in February, the Polish-born pope gave the red hat of a cardinal to two archbishops of Lviv. Lubomyr Husar, major archbishop of Lviv of the Ukrainians, is an American citizen who heads the Ukrainian Greek Catholic Church, while Marian Jaworski, a friend of John Paul who holds dual Polish and Ukrainian citizenship, is archbishop of Lviv of the Latins.
John Paul had created Jaworski a cardinal in 1998 without revealing his name for reasons never made public. Jaworski told a recent Vatican news conference that the pope never discussed the question with him.