VATICAN CITY -- In this age of a world-traveling pope the travels of church princes, or cardinals, normally draw little attention.

But when Cardinal Jaime Sin of Manila announced this week that he was flying to the Soviet Union for an 11-day visit, diplomats and Vatican analysts sat up and took notice. For with the exception of a 1979 visit by the primate of the Hungarian church, the late Laszlo Lekai, no prelate of the Roman Church has been allowed to visit the Soviet Union's repressed Catholic minority.

Though Sin left Manila Wednesday terming his trip a "pilgrimage of friendship and love," his journey was seen here as an important step in the Vatican's decades-old "eastern policy," which aims to take Pope John Paul II to the Soviet Union, possibly as early as next year.

While officials here declined to speak for the record about the cardinal's journey lest they endanger the delicate ice-breaking precedent, privately Vatican sources said they were greatly "encouraged" by signs that Soviet leader Mikhail Gorbachev's much heralded policy of glasnost, or openness, might also be extended to the Soviet Union's Roman Catholic minority.

"The visit is, of course, purely pastoral," said one Vatican official who asked not to be identified and who emphasized that Sin was traveling as the guest of the communist-influenced Russian Orthodox Church. "But even pastoral visits have an impact that goes far beyond the cathedral's doors."

What makes Sin's visit so significant is that, at the pope's urging, he requested and was granted the right to visit Catholic Lithuania. The former Baltic nation's 2 million Catholics have steadfastly resisted both Russian Orthodoxy and, since being incorporated into the Soviet Union in World War II, atheistic communism.

As a Pole and former cardinal from Krakow, John Paul II always has had a strong affinity for the Catholics of neighboring Lithuania. From 1385 to 1794, Lithuania and Poland were united, and many links between the Catholic churches of Poland and Lithuania have remained.

Having come of age himself as a young priest, then bishop, in communist Poland, John Paul II has always seen one of his key missions in the papacy as somehow freeing Christianity from the controls of communism -- in Poland, elsewhere in Eastern Europe and in the Soviet Union.

Almost from the moment he was elevated to the papacy in 1978, the Polish pontiff has sought to reinforce and encourage Lithuania's Catholics, in words as well as deeds.

In 1980, in his first consistory of new cardinals, the pope named one in secret -- an ancient, rarely used church practice for honoring those serving in a repressed, even underground, church. Vatican sources have indicated that the "secret" cardinal is Bishop Julijonas Steponavicius of Vilnius, one of Lithuania's two internally exiled bishops.

The pope and his curia lobbied hard also to visit Lithuania in 1984 for the 500th anniversary of the death of its patron, St. Casimir, and again this year for the 600th anniversary jubilee year of Catholicism in the former Baltic nation.

Both times the pope's entreaties were politely, but firmly, rejected by Soviet authorities.

The pope has not been discouraged. Despite his past failures to get invited to the Soviet Union, he has let it be known to the Kremlin that he would very much like to visit the Soviet Union in 1988 for the celebrations of the millennium of Christianity in Russia.

Last month, in his third papal tour of his native Poland, John Paul II repeatedly criticized the country's communist government, called for human rights and freedom of religion and emotionally defended the banned Solidarity labor movement. The betting, therefore, is not high on his being granted his dream by Soviet authorities.

For not only would that entail visiting the still nationalistic Lithuanian Catholics, but also paying a visit to the Roman-allied Uniates Catholics of the Ukraine, whose whole church has been officially banned and forced to operate in secret since 1946.

"The pope could not visit the Soviet Union without visiting believers in the Ukraine," one Vatican official noted. "To do that would take the Supreme Soviet lifting the ban on their existence. Gorbachev will have to be a lot stronger in the inner councils of the Communist Party in Moscow before he can even contemplate that, glasnost or no glasnost."

"Pope John Paul II sees his mission as showing the papal flag in every country in the world where there are Christians," said a Vatican official. "Being Polish, however, that means that there are some he feels are more urgent and important than others. That is why he wants to visit Russia so badly."