Crippled by Parkinson's disease but driven by both zeal and penance, Pope John Paul II will set off today to the Holy Land for the most ambitious trip he has made. It is a mission full of perils, but - if all goes well - it could be the high-point in his papacy, a landmark in the blood-soaked history of Jewish-Catholic relations, and a defining moment for Israel and the Palestinians .

As the first Pope in history officially to visit the Jewish state, he will seek to complete the reconciliation between the Roman Catholic church and Judaism which began nearly four decades ago, but which continues to be clouded by 2,000 years of persecution.

But he arrives in a landscape overflowing with rivalries between the three monotheistic faiths, small-minded prejudices, and unsettled scores.

A small foretaste of this came at the weekend, when vandals smashed the landing lights at the helipad which the Pope is to use on Mount Scopus, Jerusalem. Swastikas and the slogans "Pope Out" and "Where Were You in the Holocaust" were scrawled - in English - on the tarmac, and the Vatican's flag, now fluttering all over the holy city, was daubed with red paint. The incident was humiliating for the Israeli security services.

Although the suspected culprits are fringe extreme right-wingers, the Israeli authorities need no reminder that it was a right-wing fanatic who assassinated Yitzhak Rabin, and another -Baruch Goldstein - who killed 29 Muslims at prayer by shooting them in the back. The Pope will arrive during Purim, an ancient festival that celebrates the Jews' defiance of death (at the hands of fifth century Persians); it is also the festival during which Goldstein committed his massacre in 1994. Despite this, the Pope will be hoping for a warm reception when he touches down in Israel on Tuesday, after briefly visiting Jordan, for the 91st foreign tour of his papacy, made in defiance of his ailments and his age (80 in May). Polls show most Israelis hover between indifference and approval.

Yet the line between success and failure is thin. Pressure has been building for him to complete last week's broad-brush apologia for the errors of the Catholic Church by taking one further, politically and theologically tricky, step. The Ashkenazi chief rabbi, Yisrael Meir Lau, has all but spelt out that he expects the pontiff explicitly to ask forgiveness for the silence of Pope Pius XII during the Holocaust.

It is a difficult issue. The Pope has made penance and reconciliation his theme for the new millennium. But expressly to admit that a previous pope acted wrongly would be to undermine one of the basic tenets of the faith - papal infallibility. A specific papal apology for the church's failings in the Holocaust would also immediately trigger demands from the Palestinians for an Israeli apology for inflicting exile, misery and occupation on millions of Arabs.

On Friday, a journalist asked the Israeli government's spokesman, Moshe Fogel, whether Israel would also contemplate saying sorry to its Arab victims. An irate Mr Fogel responded that mistakes were "made by all sides", but - unlike the Palestinians - Israel was "at least capable of examining" its past."

To be fair, the calls for a papal apology are not universal. Three luminaries from ultra-Orthodoxy - which is often coupled with right-wing intolerance - appeared before the press this week to dissociate themselves.

"We are not the ones asking for an explicit apology", said Jonathan Rosenblum, of Jewish Media Resources. "In fact, we see it as a something likely to heighten tensions".

In fact, Jews tend to consider John Paul as the best Pope yet - although the competition is scarcely daunting, given centuries of being ostracised, ghettoised, exiled and slaughtered at the behest of Catholics. He was the first Pope to visit a synagogue; the first to denounce anti-Semitism as a sin; the first routinely to meet Jewish leaders.

Jewish commentators also attach importance to the fact that, during his early life in Poland, John Paul - then Karol Wojtyla - had Jewish neighbours, played in a Jewish football team, and saw first-hand the impact of the Holocaust on his Jewish acquaintances. His best friend is said to be an 80-year-old Polish Jew called Jersey Kluger.

These facts, however, are not enough to remove an undertow of resentment.

Some Jewish scholars accuse the Vatican of trying to "Christianise" the Holocaust, citing the chapels and crosses that have cropped up at former Nazi concentration camps, and also the 1998 canonisation of Edith Stein, an Auschwitz victim who was Jewish by birth but converted and became a Catholic nun.

There was fury in Rome last November when the Israeli government agreed to allow Muslims to build a mosque in Nazareth, only a few yards from the Basilica of the Annunciation - one of the world's most important Catholic churches, which marks the spot where the Angel Gabriel is supposed to have appeared to Mary (which the Pope is to visit on Saturday).

And Israelis also tend to view the Vatican disapprovingly as pro-Palestinian, not least because most of the 180,000 Christians in the region are Arab. Last month, the Vatican - keen to assert its claim to Christendom's holiest sites in its holiest city - signed an agreement with the Palestinian leader, Yasser Arafat, which denounced any unilateral (that is, Israeli) decisions on control of Jerusalem as "morally and legally unacceptable". The Israelis were livid. Claims began to circulate that the true motive for the papal visit was less do with cementing relations with the Jews than to asserting the Vatican's position in the peace negotiations.

The Pope will spend Wednesday as the guest of the Palestinian Authority, when he will hold a mass in Bethlehem, meet Mr Arafat and visit a Palestinian refugee camp. It is a big day for the Palestinians, another chance to try to convince the world that they are on the road to statehood. It is sure to be scrutinised by Israel for further symptoms of papal partisanship - just as the Palestinians will be watching the pontiff's parade through Israel with a critical eye.

In this part of the world, politics come before pontiffs.