VATICAN CITY (AP) - His shoulders are stooped and his speech is slurred, but the iron will that helped Pope John Paul II stare down communism still serves to lead his church into its third millennium.

Despite signs of growing isolation and dependence on a small group of advisers, John Paul refuses to allow his infirmities to get the better of him as he marks the 20th anniversary of his papacy.

Already the longest-serving pope chosen this century and about to become only the 12th ever with a papacy lasting at least two decades, John Paul has made his mark as the most traveled pope in history, a charismatic Pole who took totalitarian regimes to task and helped bring down communism in Europe.

His message of morality and his demand for an unswerving commitment to the teachings of the Roman Catholic Church have not changed since the day of his installation when his voice boomed out, "Open the doors to Christ.''

Physically, there is no comparison to the robust Archbishop Karol Wojtyla of Krakow who was elected to the papacy by his fellow cardinals on Oct. 16, 1978. He was the first Pole ever chosen as pope and the first non-Italian pontiff in 455 years.

To advancing age - he's 78 - and the burdens of office must be added a long list of ailments. He was shot in the abdomen by a would-be assassin in 1981. In later years he has been in surgery for removal of what the Vatican said was a benign bowel tumor, for removal of an inflamed appendix and for treatment of a broken leg.

At the Vatican, prelates openly attribute his slurred speech, trembling left hand and sometimes absent expression to Parkinson's disease, a progressive neurological disorder.

The Vatican, however, has never stated exactly what ails the pope.

"It is up to the pope's doctor to report on his health,'' the papal spokesman, Joaquin Navarro-Valls, said.

Navarro-Valls, a Spanish layman, was rumored to have nearly lost his job when he told reporters two years ago that John Paul suffered from an "extrapyramidal syndrome,'' a motor system problem that could include Parkinson's.

Vatican insiders dismiss any idea of the pope resigning if his health deteriorates, certainly not before the church completes its millennium celebrations in 2000, when millions of pilgrims are expected in Rome. The Vatican plans for John Paul to make nearly daily public appearances that year, and his 80th birthday on May 18, 2000, is marked on the Holy Year calendar.

"This pope has constantly surprised us. I expect him to continue to do so into the next millennium,'' said the Rev. Thomas Reese, editor of the Jesuit weekly America and an expert on the Vatican.

Some changes in the papacy have been visible, however.

During his vacation this summer in the Italian Alps, journalists were kept far away from the pope, who was driven on excursions in a car with darkened windows. Unlike past years, he didn't stop to bless children and wave to well-wishers.

Some bishops visiting Rome on church business say they no longer press the pope with their problems, to keep from disturbing him.

In the meantime, John Paul seems to be leaning more heavily on a few close aides, giving them their say on such things as planning the 2000 celebrations, writing diplomatic statements and organizing his daily schedule.

They include two Italians, the Vatican's secretary of state, Cardinal Angelo Sodano, and the undersecretary, Archbishop Giovanni Battista Re. Another is John Paul's Polish secretary, Monsignor Stanislaw Dziwisz, recently promoted to bishop as a reward for faithful service, which began in Krakow 32 years ago.

When John Paul elevated new cardinals earlier this year and kept one name secret, speculation fell on Re as the mystery "prince'' of the church. The pope showed his esteem when he took the entire papal entourage and security apparatus on a day trip to Re's hometown near Lake Garda in northern Italy in July during John Paul's mountain vacation.

Those who deal with the pope assure skeptics that he has lost none of his acumen and that his memory is sharp. During a trip to Nigeria in March, the pope recognized an Associated Press reporter from Rome amid a crowd of Nigerian and Vatican security men and waved a greeting.

While such high-profile trips as the visit to Cuba last January are not in the works, John Paul plans to keep up his travels next year. Destinations include Mexico and St. Louis in January, Poland in June and possibly Romania.

Frictions with the Orthodox church are holding up a visit to Moscow, while the pope's much hoped-for trip to the Holy Land seems as stalled as the Middle East peace process.

The pope has written a new encyclical, or major statement on church policy, which will be released around his 20th anniversary. It is titled "Reason and Faith'' and examines the role of religion in the modern world, a prime theme of his papacy.

In recent months, the Vatican has further cracked down on dissenting Catholic theologians, with the pope making clear he sees no room for debate on such issues as the ban on the ordination of women, the requirement that priests be celibate and the prohibition on contraception.

John Paul has always held his ground on those issues, as well as pushing worldwide campaigns against abortion.

Cardinal Joseph Ratzinger, the watchdog for doctrinal deviations, said recently that no one should be surprised at the Vatican actions. "Where have they been for the past 10 years?'' he asked.

The pope raises sexual and family issues frequently during his travels.

Navarro-Valls, the papal spokesman, said he doesn't believe John Paul will be remembered as "the pope who said `no' to condoms but as the defender of human dignity.''

In late August, as a frail-looking John Paul greeted pilgrims at his Castel Gandolfo vacation palace, one man yelled from the crowd, "There has never been such a likable and capable pope.''

John Paul responded: "Some like me, others don't. That's the way it is.''