Most of them are a remnant of Venetian and Genoese rule in southern Greece and many Greek islands (in both the Aegean and Ionian seas) from the early 13th until the late 18th century, Greeks who converted to Catholicism or descendants of the thousands of Bavarians that came to Greece in the 1830s as soldiers and civil administrators, accompanying King Otto.

Today, the majority of Catholics live in Athens, a city of about four million people; the rest of them can be found all over Greece.

Catholics can be found also in Corfu, Naxos, Santorini, Kefalonia, Zakynthos, Rhodes, Kos, Crete, Samos, Lesbos and Chios.

Since the 5th century, the Roman Catholic archbishop of Thessalonica led the Illyrian Vicariate of the Latin Church.

That year, Pope Gregory XVI established the first Catholic Church hierarchy, which was called the apostolic delegate.

In 1834, Bishop Blancis was appointed apostolic delegate and the Holy See entrusted him with the care of Roman Catholics living in Greece.