Pius was an arch-conservative who riled against modernism and confined Rome's Jews to a ghetto. John XXIII was an arch-liberal who launched the Second Vatican Council, which embraced modernism and interfaith dialogue with Jews and others.
The contrast in their personalities is mirrored in the currently preparations for Sunday's outdoor liturgy expected to draw hundreds of thousands.
At religious shops near the Vatican, photos and prayer cards of John XXIII were being snapped up, and bookstores filled whole display tables with new volumes and videos on "Papa Giovanni." "Everybody asks for him. He's the most popular pope of all," said Amelia Astrologo, who runs a religious souvenir store in the shadow of St. Peter's Square.
Two Italian TV specials were being prepared to honor Pope John, highlighting his humble beginnings, his sense of humor and his social conscience.
Pius IX, on the other hand, remained uncelebrated in Italy, despite his impending step toward sainthood. Best known for trying to hold on to temporal power, for overseeing the proclamation of papal infallibility, and for castigating modern thinking with his "Syllabus of Errors," he has not enjoyed widespread popular devotion.
Several shop owners said they've never carried prayer cards of him and that no one's ever asked for one, either.
At the tomb of John XXIII on the lower level of St. Peter's Basilica the other day, a crowd of about 40 people waited in line to kneel and pray. Flowers had been laid before his tomb, a practice that has been going on since his death in 1963.
On the other side of Rome, at the Basilica of St. Lawrence, the tomb of Pius IX was under lock and key in August, as workmen tried to solve a mildew problem.
The common wisdom about the pairing of these two "blesseds" is that it's a balancing act, an effort by the Vatican to move forward two sainthood causes that individually might provoke political opposition in the church.
During history's longest papacy, Pius IX pitted the Roman Catholic Church against a changing world: condemning emerging freedoms of speech and religion, confining Jews to Europe's last ghetto, condoning the seizure of a Jewish boy to be raised as a Catholic.
When he died in 1878, revenge-seeking Italian liberals tried to dump his body into the Tiber River.
Jewish groups are bitterly protesting Pius' beatification. Even some Catholics are challenging the church's pairing of the rigidly traditional Pius with the widely popular John XXIII, who has his own critics among conservatives for convening the tradition-overhauling Second Vatican Council in the 1960s.
"A beatification too far," a respected English Catholic weekly, The Tablet, said of Pius IX's beatification, calling it "the work of a small group of ultraconservatives."
"It can only be seen as a political move, designed to provide a conservative and reactionary counterweight to the beatification of John XXIII," The Tablet said.
"Really, I cannot understand it," said Elena Mortara, great-great niece of the Jewish boy, Edgardo Mortara, who was taken from his weeping father's arms in 1858 by papal police.
Pius "has caused so much suffering," she told the Italian religious monthly Confronti. "The wound of the Mortara case still aches in my family, and in all our community."
Church authorities took the 6-year-old Edgardo from his family in Bologna after a Catholic housemaid claimed to have baptized the boy when he appeared deathly ill. Under Pius' patronage, Edgardo grew up a church ward and later a priest.
"Even in the 19th century, actions such as the Mortara kidnapping were viewed with shock and condemnation," the U.S.-based International Jewish Committee for Interreligious Consultations said in a letter of protest to the Vatican in mid-August.
Supporters of Pius' beatification say the taking of Edgardo was justified by his baptism.
Opponents are "using today's mentality to judge the facts of 150 years ago," said Monsignor Carlo Liberati, one of the Vatican clerics who shepherded Pius' cause to beatification.
Even the current pope, John Paul II, has noted the "difficult" era in which Pius served--one in which the church often came under literal attack by the rising forces of nationalism and anti-clericalism.
Pius' 31-year papacy saw the breakup of the centuries-old Papal States, but he managed to bring the Roman Catholic Church out of the tumult intact.
In 1864, Pius issued his landmark Syllabus of Errors, 80 sweepingly negative points condemning modern ideas such as freedom of speech and religion and separation of church and state.
Point 80 rejected the modern world itself, denouncing the idea that a pope should "reconcile himself and come to terms with progress, liberalism and modern civilization."
Pius also fostered the Catholic dogmas of papal infallibility and the Virgin Mary's immaculate conception, linking them in a way that John Paul II called a "service to the faith" in 1996.
At the time, Pius' proclamation of papal infallibility prompted bitterly opposed dissenters to break away from the church--the church's last major schism until the one that followed John XXIII's Second Vatican Council.
The council went on to override much of Pius' Syllabus of Errors in an effort to make the church more accessible to ordinary people. The innovations included allowing Mass to be celebrated in local languages rather than Latin.
The liberalization John XXIII set in motion make him a mistaken leader in the eyes of many church conservatives.
In Italy, however, Catholics are hanging crepe ribbons and balloons to celebrate his relatively quick-paced beatification. The peasant-born John is still known here as simply "The Good Pope"--more for his kindly manner than for any matter of church doctrine.
Ironically, John XXIII's cause for beatification long had been counterbalanced with that of another conservative Pius, the World War II-era Pius XII.
But Jewish groups increasingly protested what they said was Pius XII's silence against the Holocaust, and church officials quietly let it be known this year that the wartime pope's beatification was off the calendar for 2000.
Pius IX's, with its then less-known Jewish issues, moved up.
Ironically, however, John XXIII favored Pius IX's sainthood cause and considered him a truly holy man, according to John's former secretary, Archbishop Loris Capovilla. In 1961, in fact, John XXIII spoke at a general audience about the possibility of seeing Pope Pius canonized one day.
Pius IX reigned for almost 32 years, the longest pontificate since the days of St. Peter, spanning a period of intense church-state tension in Italy. Pope John, elected at age 77, ruled for less than five years. He was chosen as a transitional pope, yet he ended up recasting the church's relationship with the modern world by breaking free of the Vatican city-state in a systematic way, making more than 140 trips to jails, orphanages, churches, and schools.
John XXIII also wrote groundbreaking encyclicals, including "Mater et Magistra" [Mother and Teacher] on Christianity and social progress and "Pacem in Terris" [Peace on Earth] on the need for global peace and justice. And he let it be known that the church was not afraid of science or its discoveries, and he sparked an ecumenical revival by reaching out to separated Christian churches.
After Pope John died during the Second Vatican Council, some participants wanted to proclaim him a saint by acclamation, thus giving the world a sign that the church did not consider him a "dreamer." That idea was shot down by the Roman Curia, and his sainthood cause slipped into the Vatican's painstakingly slow process of verification and documentation.
In linking the beatification of the two popes, Pope John Paul may want to highlight a certain continuity between the First Vatican Council, which was called by Pope Pius, and the second council.
For the church, sainthood goes beyond personality traits and papal policies. When these two popes are beatified Sunday, perhaps Pope John Paul II will offer some insight into how such different figures can both be deemed universal models of holiness.