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.