Though some 98% of Greeks are born into Greek Orthodox families (regardless of how religious individuals in those families are), there is also a sizeable Roman Catholic minority, mostly in Athens (many of whom are Filipino and Polish immigrants) and in the Cyclades especially on the island of Syros, where they make up 40% of the population) but also in Naxos and Paros. All of the Cyclades constituted a Venetian duchy from the 13th to the 16th centuries.
Before World War II there were some 80,000 Jews in Greece, 60,000 of whom lived in inThesalloniki, with all but about 10,000 remaining after the war, most of them shipped off to their deaths in Auschwitz.
Some 46,000 of them were Sephardic Jews, the first of whom came to Greece from Spain in the 15th century when non-Christians were expelled during the years of the Inquisition, and who formed a majority of the population of Thessaloniki for many many years. The Turks had welcomed them with open arms because as Jews they were "people of the Book" and the city was under populated.
Earlier communities of Jews, termed Romaniot Jews (who came to Greece even before the Roman times for which they were named), were scattered in many areas in Greece, some of them dating back to 300 BC. There are about 5000 Jews in Greece today, 3000 of them in Athens, 1000 in Thessaloniki, and the rest in small communities here and there, mostly on the Greek mainland.
There are about 120,000 Muslims in Greece, residing in Thrace, where they make up perhaps as much as half of the population. They are made up of Turks, Pomaks (a race speaking a dialect of Bulgarian, who were forcibly converted to Islam centuries ago), and Roma (the politically correct name for people formerly known as gypsies), though the larger numbers of Roma residing in Greece outside of Thrace are non-Muslim.
Muslims were permitted to remain in Thrace after the population exchange of Christians and Muslims between Greece and Turkey referred to above ( in 1923), and there are smaller numbers of them on the islands of Rhodes and Kos in the Dodecanese islands.