He is likely to be met by protests from Greek Orthodox Church hardliners who refer to the 80-year-old Pontiff as "the arch heretic" and "the grotesque, two-horned monster of Rome".
Following in the footsteps of St Paul in Greece, Syria and Malta, the Pope's arrival in Athens marks the beginning of one his most controversial overseas visits in almost 23 years of travelling the world.
He is likely to be met by protests from Greek Orthodox Church hardliners who refer to the 80-year-old Pontiff as "the arch heretic" and "the grotesque, two-horned monster of Rome".
Parish priests in Greece are talking about flying black flags and ringing their church bells as a sign of mourning, while security around the Pope will be very tight.
Greek Orthodox leaders have reluctantly accepted the papal visit - only because it is a pilgrimage, they say - but they want the Pontiff to use the occasion to ask for forgiveness, as he did with the Jews.
Ancient hostility
The BBC's Paul Wood in Athens says that although such an outright apology is unlikely, many expect the Pope to issue a statement of reconciliation.
The hostility towards Rome dates back a thousand years, to the Great Schism of 1054 that split the Christian church into Eastern and Western branches.
With about 98% of the population baptised in the official Orthodox faith, much of that antipathy remains alive.
Greek Orthodox followers regard theirs as the true faith, and accuse Rome of attempting to impose its supremacy over the Christian world during past centuries.
Mosque visit
The Pope is to meet the head of the Greek Orthodox Church, Archbishop Christodoulos, during his two-day stay in Athens, but no joint prayers or theological discussions will be held, according to a church spokesman.
They are expected to visit together the hillside where St Paul preached to Athenians in the first century.
Our correspondent says it may well be the Pope's most ambitious attempt to heal the historic rift with the Orthodox church.
He will also be attempting to reach out to other faiths in his next stop - Damascus, the scene of St Paul's conversion, where he will become the first pope to set foot inside a mosque.
The Vatican says it will be the first time that Muslims and Christians have prayed together in an organised way.
He will return to Rome via the staunchly Catholic island of Malta, where St Paul was shipwrecked.