ROME — In the earliest days of Christianity, St. Peter and other leaders of the new Church routinely attended Jewish places of worship. In fact, there was a rollicking debate as to whether one could even be a Christian, let alone a pope, without doing so.

Before long, however, Christianity broke with Judaism, and it would be another 1,900 years before another pope would cross the threshold of a synagogue. In the past 30 years, if such visits haven’t quite become routine, they’re nevertheless comfortable and almost expected.

On Sunday, Pope Francis will make his first synagogue visit as pope, traveling across Rome to stop at the city’s historic Great Synagogue, located within the boundaries of what was once Rome’s Jewish ghetto.

The outing falls on an evocative date, as Sunday is an annual Italian day for Christian-Jewish dialogue. It’s been observed for the past 20 years to reflect on relations between Catholics and Jews, and to recall the Jewish roots of the Christian faith.

A similar day of dialogue is celebrated in other countries across Europe, including Poland, Austria, the Netherlands, and Switzerland.

Roman Jews also recall Jan. 17 as Mo’ed di Piombo, marking a day in 1793 when a massive rain, which tradition regards as miraculous, saved the Jewish community from a pogrom.

However many papal visits to synagogues there end up being in the course of time, there will forever be only one modern first. For that reason, St. John Paul II’s groundbreaking outing to Rome’s Tempio Maggire in 1986 probably will always be the most memorable.

John Paul II grew up with Jewish friends in the Polish city of Wadowice in the 1920s and 30s, and was profoundly affected by the horrors of the Holocaust. Although his papacy was not free of controversy with Jews, including a debate over the erection of a massive Cross at a Carmelite convent in Auschwitz in the 1980s and the canonization of Edith Stein in the 1990s, he was nonetheless seen as an historic pioneer in relations between the two faiths.

In 1979, John Paul II became the first pope to visit Auschwitz, kneeling down to pay his respects. His 2000 visit to Israel, when John Paul placed a handwritten note in Jerusalem’s Western Wall apologizing for Christian anti-Semitism, was described by the rabbi who was his host as “beyond history, beyond memory.”

Thus when he arrived at the Rome Synagogue on April 13, 1986, it was seen as another revolutionary gesture from the Polish pope.

Sitting in an armchair next to the legendary Elio Toaff, at the time the chief rabbi of Rome, the Polish pope addressed Jews as “dearly beloved elder brothers.” (Toaff died in 2015.)

During his remarks that day, John Paul II denounced a society lost in agnosticism and individualism, while suffering the consequences of selfishness and violence. He described Jews and Christians as the trustees of an ethic marked by the Ten Commandments, hence making the promotion of a common reflection and collaboration “one of the great duties of the hour.”

The visit was engineered by Argentine Cardinal Jorge Mejia, another pioneer in Catholic/Jewish dialogue. (Mejia died in December 2014 at the age of 91.)

According to an interview Mejia gave in 2009 to L’Osservatore Romano, the Vatican’s newspaper, the possibility of the visit to the synagogue was first considered during a working lunch with John Paul II to prepare a papal trip to the United States planned for 1987.

The pontiff wanted to visit a synagogue during his tour, something no pope since the earliest days of Christianity had done. Mejia replied that such a historic papal first should happen in his own diocese of Rome.

“John Paul II asked me if, in my opinion, that was possible,” the late cardinal told the newspaper. He then proceeded to call Toaff to present him with the idea.

According to Mejia, the rabbi responded by quoting Psalm 118, “Blessed is he who comes in the name of the Lord.”

(Another version of the same story has it that the conversation with John Paul II about the synagogue visit came with the late Italian Cardinal Roberto Tucci, who planned most of the pontiff’s overseas trips.)

John Paul’s successor, Pope Benedict XVI, chose his home country Germany for his first visit to a synagogue. It came in the early months of his pontificate, during a visit to Cologne in 2005.

This visit had a special resonance because of Benedict’s past: Involuntarily, the future pope had been briefly enrolled in the Hitler Youth. During World War II, he was forced to serve in the German army, from which he deserted before reaching the age of 18, briefly ending up in an American POW camp. (As a footnote, Benedict was thus the first pope in history, and so far the only one, ever to have been an American prisoner of war.)

During that visit, Benedict called the Shoah, the preferred Hebrew term for the genocide of Jews by the Nazis, “an unspeakable and previously unimaginable crime,” and warned against the rise of new signs of anti-Semitism.

His second visit to a synagogue came in 2008, during his trip to the United States.

It wasn’t until 2010 that Benedict visited Rome’s Great Synagogue, and the stop was overshadowed by controversy. It came barely a month after he’d approved a “decree of heroic virtue” for Pius XII, advancing the sainthood cause of the wartime pontiff whose alleged silence on the Holocaust has long fueled disputes.

(The decree allows Pius to be referred to as “venerable” and leaves him two miracles away from being canonized.)

“The silence of Pius XII on the Holocaust is still painful,” said Riccardo Pacifici, then head of the Jewish Community of Rome, in his remarks to Benedict XVI during the visit.

“Perhaps he could not have stopped the trains of death, but he could have transmitted a signal, a final word of comfort, for our brothers on their way to the camps of Auschwitz,” Pacifici said.

Rome’s chief rabbi, Riccardo Di Segni, was more indirect, but no less clear in his reference to the divisive pontiff.

“The silence of God about the evils of the world, or our inability to hear his voice, is an inscrutable mystery,” Di Segni said. “But the silence of man is a different matter. It confronts us, it challenges us, and it does not escape judgment.”

Speaking to journalists earlier this week, Di Segni, who will also welcome Francis on Sunday, reiterated those remarks when asked about Pius XII, and said he hoped the late pope remains “absent” during the visit.

On Sunday, Francis is expected to pray at a plaque recalling the events of Oct. 16, 1943, when German troops occupying Rome stormed the Jewish Ghetto, rounding up more than 1,000 Roman Jews, including 200 children, and eventually deporting them to Auschwitz.

Only a handful, 15 men and one woman, ever returned to their homes.

The pope is also expected to pray at a monument commemorating an Oct. 9, 1982, attack on the Great Synagogue of Rome perpetrated by Palestinian terrorists.