St. Peter’s Square in Rome will fill with admirers, church bells and applause this afternoon as Pope John Paul II celebrates a special thanksgiving Mass marking the 25th anniversary of his towering pontificate.

“Just by virtue of the length of his papacy, he casts a shadow that will reach into the next century,” said Chester Gillis, professor of theology at Georgetown University.

His is already the third-longest pontificate in Roman Catholic history, and more than half the world’s Catholics have grown up never knowing another pope.

Although his jubilee arrives amid mounting concern for the 83-year-old pontiff’s health, the once-vigorous, globe-circling leader continues to put his stamp on the church.

On Saturday, John Paul is scheduled to meet with most of the world’s cardinals. On Sunday, he is to beatify Mother Teresa of Calcutta.

And on Tuesday, he will convene a meeting of cardinals to formally create 31 new cardinals, including Philadelphia’s Archbishop Justin Rigali.

Over the years, he has used his international pulpit to promote social justice, reach out to other faiths, affirm the sacredness of all human life, and to condemn war, materialism and a “culture of death” that condones abortion, capital punishment and euthanasia.

Pope John Paul II visits with children in traditional costume during the weekly general audience at the Vatican.The pontiff presided at the audience before tens of thousands of pilgrims who packed St. Peter's Square on Wednesday to pay tribute to the ailing pope for the 25th anniversary of his papacy.

His hierarchical style of leadership and some of his orthodox stances have alienated large numbers of Catholics in the developed nations, however.

He has asserted, for example, that the church may never ordain women, that homosexuality and other sexual activity outside marriage is sinful, that divorced-and-remarried Catholics cannot take communion.

A survey 1,508 Catholics by LeMoyne College of Syracuse, N.Y., in 2001 showed that 90 percent approved of John Paul’s leadership but that a majority disagreed with his bans on artificial birth control, married and women priests, and his condemnations of homosexual activity.

“The poignant paradox of his tenure is that most American Catholics adore him, but they have a very hard time with his message,” said David Gibson, author of “The Coming Catholic Church,” about tensions in the American Catholic Church.

Church membership has nevertheless grown under John Paul from about 700 million to more than 1 billion, with the greatest expansion in Africa and Asia. “That will be a major part of his legacy,” said Peter Phan, a Georgetown University professor who studies Asian Catholicism.